I am running an Ms Paint Adventures inspired adventure in their forum so far with some light rationality themes: Blockstuck.

I am also giving thought to designing a lazy-evaluated LISP derivative with CAS features, DSL for assembler JIT, category theoretical constructs, innocations in type systems, etc. So far not much in terms of code, since I lack a good language to implement it in. C++ has load of libraries but no GC, D has good GC but lacks libraries.