Here are more ideas I pulled out of the Rust issue tracker.
Add compiler-inserted yield checks - task scheduling is totally unfair, and part of the problem is that tasks don't yield automatically yet unless they are using library functions that must yield for correctness. https://github.com/mozilla/rust/issues/524
Fix debug info. Rust code can't really be debugged in gdb because it doesn't emit the proper DWARF info. This is mostly about applying the LLVM APIs correctly, but will involve low-level debugging. Fun stuff.
Add a task monitor. This would probably be a named service that can be queried for the task state, can provide messages on task lifecycle events. Also give tasks useful names.
Here are more ideas I pulled out of the Rust issue tracker.