Right now we're working on the microbe stage, and when we get to creature parts there will be a lot of flexibility in terms of scale, shape, dimensions, etc. So we shouldn't jump into that quite yet. If you want to help with organelles, @TheCreator is the graphics team lead, he may have something for you. I think we need a model for the agent secretors
If you truly are eager to make creature parts, I'm in the process of making the new intro for Thrive, so you could create entire creatures, vehicles, or anything else you can think of for it. I especially need aquatic creatures, anything helps really, even trees and ferns, but remember, try to make them non-earth like. You have complete creative freedom here, the more people creating assets for the intro the more variety the scenes will have.