This would enable private spaces to actually be private, e.g. a Topi couldn't just walk through the wall of someone's office. They would need to enter through the doorway.

This is a must for me. And while it's true there are potential issues, I think most of these could be solved by having impassable objects toggleable by the world creator and having that toggle set to 'off' by default. This is not something that will be needed for many worlds, but for those that want to offer more immersive exploration or game-like experiences, this one feature will open up whole new worlds of possibility.

If there is collision attached to a topie then topies could be pushed by other topies. Not sure you want that. Mike

This would help with immersion but might cause problems with the topia figure getting stuck places