There are two interesting project ideas that I would like to try my hand at eventually, as I get more free time:
1) Closures for C? It's an interesting idea that is being developed elsewhere. I would love to find a way to add this, as some kind of optional parser mixin, to Parrot's C implementation. I could copy the syntax that's being used for this in Clang and llvm-gcc, to increase interoperability with those projects
2) Alternate Syntax for C++, which has been developed by none other then Damian Conway. I would love to see this implemented as an alternate parser for C++, which could share almost everything (except the parser) with a "standard" C++ implementation. Implementing both in parallel would be very fun (and if I could combine with #1 and give C++ some closures too, that would be hot).