Call me selfish, but I treasure programming languages, specially the weird and unconventional ones, and specially if they have some sort of academic/theoretical brownie points.

You have already done BLISS, I have been meaning to clone it ever since I read an old rant by Dennis Ritchie on usenet. I am weirdly attracted to CMU languages, so I am gonna say Dylan :-) Marlais is open source and you can use it as a starting point. I am half way through cloning DUIM for Common Lisp and, frankly, all their "syntax" innovation would be lost to me. I am forever ruined for non-Lisp syntaxes, I think. ISO Lisp would be cool too.

If you weren't a compiler hacker I would have recommended Oberon.

For some reason, I could never see compiler hacking outside the traditional code generation for a real machine. If I wanna do source translation I would probably stick to macrology.

Miranda would be cool too, specially since there are no FLOSS compilers for it.

Mozart/Oz has a tiny kernel which you can clone, but I don't want you to learn this in that matter. Instead, get the book by van Roy and Haridi and enjoy at a leisurely pace.

Someday I would love to implement an APL and a Prolog dialect. Someday.