Oh wow there are 2 people working on plugins, I feel like I'm late to some sort of plugin party.
I hadn't really thought about it until yesterday either, but you can see why it'd be super useful, I'd actually pay money for that kind of feature, its up there with mesh deformation for me.
The collider thing would be sort of strange I guess, especially with everyone implementing different collision methods within their game, and using a box collider on a foot wouldn't be wise at all, though I believe its usually done without colliders, theres just something else that detects the terrain (raycasting I guess, I'm not much of a coder, all artist here) then throws the angle back to the bones and the bones move accordingly, or at least this is my understanding of it from using previous IK systems, including the old unity one.
I hadn't really given much more thought than the legs honestly, and the arms too of course, I was recently playing LIMBO and noticed that it happens there, along with reaching out to grab-able objects if they're within a certain distance, figured IK was the only possible way really. Its great to hear you're open to the idea though.