A genetically modified axolotl attached to an amputated limb of a mammal could be used as a living bioreactor to regenerate the limb. In this case, it would serve a similar function as a placenta.
This idea was inspired by this paper, this tweet, and this idea.
Ultimately, a way to regenerate limbs in mammals (someday in humans). Until then, this sounds like an interesting experiment we could learn from and iterate.
Axolotls are able to regenerate limbs, internal organs, and even brains. One of the factors that make this possible in axolotls, but not in mammals is a different environment that forms around the amputation wound.
What if we could use the axolotl to "lend" us the right environment and all the necessary factors? What if the axolotl served as a live bioreactor that is attached over a freshly sliced amputation spot? It would be kept alive while the amputated mammal limb grows inside it.
To prevent the different immune systems from killing each other's cells the axolotl could be genetically modified to express the patient's (mammal) haplotype. Or/and it could be a chimera where during the axolotl's development stage, the mammal's induced pluripotent stem cells were inserted.
Being sewn onto a damaged axolotl, human cells would find themselves in the right environment where regeneration is possible (due to axolotl cells chemical signaling, etc). Rather than producing a scar, human cells would begin regenerating - just like they did during embryogenesis. They know how to do it. They've done it once before. The axolotl environment just makes it possible again.
Now... how to prevent the axolotl and human cells from fusing together without regenerating either. I don't know. Maybe someone knowldegeable could make this idea feasible?