I have a lot of side projects at various phases of investigation right now. So many, in fact, that I'm quite sure that I won't get around to actually implementing even half of them. Because completed projects are more important to me than the right to say that I wrote the code, I'm going to list some of them here in the hopes that someone on the lazywebs will be inspired to do something with my vague description.
Add New Features to Clang. The Objective-C runtime (at least Apple's version of it) allows any string to be specified as the name of a class, but existing Objective-C compilers only allow letters, numbers, underscores and dollar signs in class names. I'd really love to introduce the ability to have class names with greater variety. Apple has already shown their willingness extend the Objective-C language; I'm just going one step further. Here are a few more ideas that I'd like to see in clang:
Refactor Monodevelop. Monodevelop has great support for things like syntax highlighting and code completion, but most of these features are very tightly tied to Gtk#, and if you want to access such features using a different UI framework, you're out of luck. I'd like to see Monodevelop refactored so that there is a library of common, platform-agnostic code that could conceivably be used from a variety of applications. I know I'd use it.
Write an Objective-C interop library for .NET that uses IDynamicMetaObjectProvider. Objective-C is a very dynamic language, so this sort of implementation makes quite a bit of sense. Individual Objective-C objects would likely inherit directly from SafeHandle and implement IDynamicMetaObjectProvider; all/most method invocation would happen via DynamicMetaObject. Then, using C#'s new dynamic keyword, most of the functionality in Cocoa would be accessible without the need for an enormous wrapper library.