instead of creating and gestating complex organisms, you could have reengineered bone structures that are more easily shifted around on a short timescale.
Reengineered bone structures that may be more easily shifted around on a short timescale is just the shapeshifting boon I had in mind for the Bio/Nano combo. ;-)
I had several ideas for easier shapeshifting.
First off, I had the same thought of a Bio 5/Nano synergy. You could have techno-organic hybrid cells, like mitochondria. They'd kinda act like a cross between pluripotent stem cells and an amoeba. And they could have much more advanced communicative and computational abilities, allowing them to respond much faster and more precisely to any events in the body and coordinate better.
How fast could something like that be? I dunno, but I'm assuming it wouldn't be enough to make instantaneous shapeshifting possible.
Now, the next idea I had was taking the intelligent sponge biocomputer from GURPS and crossing that with the idea of a vasculoid which replaces the need for a heart.
Then I had the idea of some sort of advanced smart skin that's kinda like a modular slime mold, or a weak form of programmable matter in biological form. It could rapidly harden in a super-tough form like liquid armor, change colors like an octopus or emit lights, and each piece of it would have some degree of mobility and could change shapes like a slime mold.
Then I thought, well couldn't the vasculoid brain-sponge thing rapidly become this smart skin too? Why not. Maybe it could even replace the need for traditional bones, sort of.
Then, assuming that organs besides the heart and brain might still be needed, maybe they could have a sort of modular redundancy, so each organ could rapidly take up the role of another organ, and they could rearrange themselves in the body as needed with an internal system. Should be able to form organs from the vasculoid also, but I'm not sure how fast.
As for the skeleton, aside from maybe being able to become like a slime mold, maybe it could be like the skin, being separated into interlocking hard plates that might be shifted around into different arrangements quickly.
But all this kind of has the underlying assumption that you want to be a shapeshifting human-identified person that can "disguise" themselves. But what if you take these ideas in a different direction, you're like an uplifted mimic octopus that wants super-enhanced mimicry abilities for the sake of art or something? I feel like that's more interesting, and you might be to do rapid "shapeshifting" that's not really a convincing disguise but simply aesthetic. Or maybe you change to look similar to a variety of forms just to relate closer to people.
Sorry for rambling, just like making up stuff like this.