If creation requires death and dismemberment to occur, then it would follow that only the sacrifice of something living will do to fulfill a cosmological sacrifice. This is not to say that monetary and other sacrifices cannot be made under other circumstances. They obviously were, and from what other folks here have said, this method is still being used, although it is in the context of a gift to the Gods rather than of cosmic renewal. Mauss would say that this sort of sacrificial gift creates a mutual relationship between the Gods and the human community that requires a reciprocal gift from the Gods of continued food, shelter, and other necessary survival substances. But as I've said, these gift exchanges do not renew the cosmos in a theological sense. They serve instead to renew community bonds. An important task to be sure, but not the point of cosmological sacrifice.
Some anthropologists and historians have speculated that the sacrifice of animals followed a period of the sacrifice of humans as the vehicle of cosmic renewal. We do know that the Celts sacrificed prisoners of war and occasionally other humans in some rituals, so they had not left that phase of sacrifice behind them entirely. I think that in this case, what we may be looking at are gifts to the Gods, or an exchange of life for life on the battlefield in the case of prisoners of war. Hypothetically speaking, the warriors of "our tribe" were successful and few were killed, but war is an arena of death and certain loss of life is expected or perhaps vowed as a part of the victory celebration, so prisoners from "their tribe" are sacrificed as a substitute for "our" warriors or as gifts to the deity of warriors. Other human sacrifices may serve as messengers to the Gods, carrying requests and information that cannot be trusted to lesser gifts. A human sacrifice, particularly as a foundation sacrifice, may serve as a spiritual guardian for the structure being built. But at some point, animal sacrifice was apparently substituted for human sacrifice in cosmic renewal ceremonies, as well as in other kinds of sacrifice, and so there would seem to be precedent for considered changes in this kind of ritual. We are not, then, looking for "an excuse to stop performing the sacrifice" but rather a theologically valid way to transform the sacrifice while maintaining its focus and impact, as was done in the alleged transition from human to animal sacrifice. I believe that we can argue for a theologically valid substitute for the body and soul of an animal.
We know from the story of Miach and Airmid, and from Alexei's account of Breton herbalism, that herbs are associated with different parts of the body -- an herb for every joint and sinew, as it were. We might say that the body could be created, built of herbs. Blodwedd is an example of a living human being magically created from nine kinds of herbs. We also know from a Welsh medieval medical text, and from Irish tradition, that the body is related to the cosmos in Celtic thought. The eyes may be the stars, sun be the face, breath be the wind, stone as bones, water as blood, soil as flesh, etc. I would argue that through these associations, a living "human" body could be created of certain ritually appropriate plants to serve as the vehicle of cosmic renewal. In this way, the death and dismemberment of the "herbal body" would serve as the living force that is the source of cosmic creation.
The sacrifice of plants would not be invalid as a sacrifice of spiritual efficacy. Plants have sentient spirits, no less than humans or animals. In fact, I would argue that plants are more important than humans or animals in a biological sense, for without plants, the energy of the sun cannot be transformed into food, and no life would exist without them. Plants can exist without us, but without them there is no possibility of independent animal life. The life force of a plant may, therefore, be considered just as pure and acceptable for sacrifice as would the life of a cow, or a human being. From the point of view of primacy, it might even be considered more acceptable, more pure, as they are closer to the primal source of solar energy than we. Individual plants lack only the social construct of the complete human body to be an acceptable substitute for the human being. But as we have seen, this can be attained through ritual.
The question then becomes, what plants would be appropriate as sacrifices? How many, and what must they symbolize? There are several ways we can approach this question. One way is to examine the list of nine herbs from which Blodwedd was created, knowing that they served together to create a living human being. Nine, of course, is a primary and significant ritual number among the Celtic peoples. It is a number of wholeness and completion. I believe that regardless of what criteria we use to establish a list of appropriate plants, they should be nine in number.
An approach we could use toward choosing the appropriate plants is to look at their functions. As an example, plants are used for food (oats), for medicines (foxglove), for fiber (flax), for fuel (alder), and for their mind altering properties (belladonna). We could examine the herbal lore of the Celtic peoples and choose plants from these categories that also represent the various parts of the body to use as the sacrifice. Plants used as entheogens would seem to me quite appropriate for use to symbolize the soul of the cosmic being, for instance, or to symbolize the head which is the seat of the soul, and the vault of the heavens. Food plants might symbolize the flesh. Medicines would serve to symbolize many different parts of the body, as they are used to treat the different organs and body systems. Fiber plants might symbolize hair, or the skin covering the body. Fuel plants might represent the spark of life, but they would of course be used to burn the sacrifice during the ritual, so perhaps they might not be necessary to the construction of the herbal body.
In doing this kind of a sacrifice, I believe the plants would have to be fresh. The life force would have to still be within them in order for the sacrifice to be useful or valid. Going out and buying dried roots and leaves just would not hold the same energies as growing or wildcrafting the appropriate plants. It is not a sacrifice of life and spirit if the spirit and the power of the living green has gone out of the plants six months ago. Lack of spirit would invalidate the sacrifice as a meaningful ritual, meant to connect us with the necessity of an immediate and felt death in preparation for cosmic creation. There is little emotional impact in burning the dried dust of leaves, while the burning of freshly harvested plants connects us with the death of the green being. The sap of life should still run through them, preferably having been harvested in a ritual manner no more than a day in advance of the sacrifice. As with the cosmological sacrifice of an animal or a human being, the plants used must be free of blemish, so that the world when renewed will be likewise perfect in body and potential.
Although Celtic lore regarding plant associations with the body is essential knowledge, I believe that we also have to consider the implications of attempting to sacrifice plants not grown in native soil, aside from the difficulty of obtaining fresh non-native plant materials. What local plants carry the same symbolic significance as the ritual plants of Celtic Europe? As modern Celts, scattered all over the globe and born of many different ethnic backgrounds, I believe we must renew the local cosmos through the use of locally grown plants for this kind of sacrifice. This would necessitate close observation of local plant lore, and the development of relationships with local plant spirits so that we begin to understand how they fit into the herbal body. Blodwedd's creation would be different in Minnesota than it was in Wales, different again in Sydney or Vladivostok. If our spirituality includes a deep and true connection with the land on which we live, we must take these things into consideration and develop a theology of place and of what Gary Snyder refers to as reinhabitation. The essence of the soil where we live must enter us physically and spiritually, must link us with the spirits of the place.
Some ritual would need to be devised to take the chosen nine diverse plants and "birth" them together as one living being, whole in the sense of a cosmic and divine body. It would have to transform them from a group of separate plant-beings into the divine body of the sacrifice, to unify their spirits into one perfect soul. This, of course, is not necessary in the case of animals or humans, because they are already a single, discrete life force and body. The spell that created Blodwedd must be spoken, as it were. From that point, the sacrifice could proceed as the ritual of death and dismemberment, with the burning of the plants to release the spirit-body and its component parts in the act of cosmic renewal.
The next question is, who deals with this kind of sacrifice. In Celtic society, Druids claimed that they created the universe, and through this form of sacrifice it could be said that they indeed do so. Druids acted as sacrificing priests according to the Greeks and Romans who encountered them in Gaul, and were capable of excommunicating people from sacrifice if they violated the laws of the tuath. Among the Hindus, it is the Brahmans who perform these sacrifices. Is cosmological sacrifice really something that can be performed in an egalitarian fashion, with any celebrant off the street capable of playing the part of sacrificing priest/ess? Will anyone who has been ritually purified be acceptable as the sacrificer? Should the sacrificer be one who has not killed in battle? One who has not violated certain laws of the land? If people could be excommunicated from participation in the sacrifice as observers, it would stand to reason that some crimes would certainly disqualify people from actually making the sacrifice. If these things are important, should the one who performs the sacrifice be required to be someone who identifies as a Druid? Someone who has at least studied Celtic lore and theology enough to understand the implications of sacrifice and renewal? If it is an animal rather than an herbal sacrifice, it is also essential that the sacrificer be skilled in the humane killing of animals so that the sacrifice does not suffer unnecessarily. It seems obvious that some training would be required for the effective sacrifice of an animal, at the very least.
What should be the ritual state of mind required of the sacrificer? Obviously there is more going on here than just going through the motions. To perform the ritual without the proper intent and the proper state of spiritual purity would be more sacrelige than sacrifice. To attain the proper intent and the proper state of purity is probably the work of many years of training and meditation, and a certain amount of asceticism. At the least, one must be able to perform the ritual while maintaining the required state of mind and visualizations without falling prey to distractions. The outer form of the ritual must also be done flawlessly, without reading from 3x5 cards. The intent and focus of the assistants and witnesses to the ritual must likewise be pure, although perhaps not as flawless as the state of the one who must physically perform the sacrifice. Anything less endangers the very renewal of the cosmos, if we follow the logic of the ritual itself. If we take this at all seriously, these considerations argue against the idea that anyone who feels like it can perform a cosmological sacrifice, whether animal or herbal, without first intensively practicing both the outer and inner techniques and studying the theological implications.