We intend to use a number of different programming languages in the STEPS project. Even right now, if we only look under the hood (in what we call the "engine room"), we're already using three: Pepsi, Coke, and an OMeta-like language for parsing. Coke plays a special role among these languages, since the semantics of the other languages are defined via translation into Coke.
One thing programmers—especially Smalltalkers—really care about is debugging. Coke has recently started to get some debugging capabilities, and we can expect that these will soon be on par with (hopefully even better than) Smalltalk's. This is definitely a good thing, but we can't stop there; the other languages also need support for debugging, and making this work in under 20K LOC is not a trivial task.
Scheme programmers often implement little DSLs using macros, so I hope that we can get some inspiration from Dave Herman's debugging library for PLT Scheme, which seems very interesting.