I've had in my head for a while an idea for a programming language inspired by John Shutt's Kernel. But, I go even further than merely first-class macros and environments. I give you: first-class (well, there really isn't a word for this, so I guess for now I'll call them) transframes (or xframes for short). Basically, first-class operators that can modify the evaluation-stack.
I've done some work on this in the past, making an interpreter and such, but I'll be rebuilding it, so I can document my thought process as I go. It's gonna be inefficient as heck, but I'm curious to see where is leads.