It would be extremely interesting to have a collage feature.
Many artist (including myself some time) use different papers in the artwork and it would be very nice to have features such as and addictional paper texture so if you paint over it you see the different depth, paper both cut or torn (they create two different effects), different pattern and images....

This is an example of what I'm saying:
(artist here)

I guess technically it's just an extension of the stamps but with a couple of features more (such as cutting the figure, the texture, maybe some folds....), but it would be really fun.