I often find myself in need of writing down a future Visual Studio solution improvement. Having them at hand while working on personal projects is very handy and accelerating.
Is there an extension (or maybe even a build-in feature) that would allow me to write down future changes to a list, and mark rows as "done" after implementation is finished?
Yes. You can use the "Task List" in Visual Studio:
Menu View → Task List
Also, in the code, you can use tokens in your comments, so Visual Studio can auto-populate your task list. Such as:
//TODO: xyz
//HACK: ABC
The predefined tokens can also be extended:
Visual Studio → menu Tools → Environment → Task List → Tokens