On the topic of gears, the 2d curve collisions in Kangaroo recently got a serious update:
Rigid curve collisions
This works with native Rhino curves directly, so no need for any meshing or discretization, and is there to use now.
The 3d rigid body constraints have also been updated, fixing a serious stability problem that existed before:
swashplate simulation
3d collisions between rigid bodies are still not great, but I’ve got an update for that in the works too. I wouldn’t expect it to be able to handle collapsing stacks of thousands of boxes in realtime - Bullet or PhysX are still your best bets for that kind of thing, but for things like making sure the parts of your mechanism don’t pass through each other I think it can be enough.
Soft bodies in Kangaroo, both 2d and 3d are continuing development, with an emphasis on customisability and accurate elastic deformations.
Whatever happens with Bongo, my plan is still to make the Kangaroo solver accessible directly in Rhino.
Something I think could allow very flexible ways of making animations (using whichever solver is most suitable depending on the task) would be a simple timeline editor for Grasshopper. Just basic keyframes and curves, and a way to link those values to Grasshopper parameters. Of course you can do this now with complex collections of linked sliders, but it’s rather cumbersome if you want a sequence of multiple movements. A unified timeline could make a big difference.