Procedural tree generatior that actually is procedural all the way (no providing leaf textures, possibility to create everything in geometry including the leaves and their textures, no texture / models provided for anything, just numbers sliders and nodes as input and a full tree as output)

Write a scriptable file watching daemon.

It's common in game engines to dynamically reload files as they change. Typically, they're either written in-engine or extremely specialized for the task. The open source ones are mostly batteries-not-included single classes. What would probably be ideal is one that supports a plugin interface for both subscribing and publishing, so that users can provide their own DLLs to govern implementation for their mechanism of choice.

This is probably the only "that would be really handy" thing I've ever seen where someone hasn't already said the same thing and written it themselves. It's been on my list of neat things to do for some time.