Always handy to have a list of stuff other people find handy in programming languages.
http://repinvariant.blogspot.com/2008/01/thoughts-on-lisp-things-that-other.html
personally i agree with the bignum point, pluk's default int implementation automatically starts to use bignums instead of int when an int overflows, at least that is what i want to implement if i get that far :)
Tail recursion as an optimization might be fun to implement, but what would the call stack look like, also how to detect this at all.
Multi methods is something i don't have a clue about, not sure how to put it into syntax, how to implement it efficiently, what to use it for.
D also has a lot of interest for me, pluk is somewhat similar to it.
http://www.unknownbrackets.com/tutorials/polishing-d
I'm not sure how D will ever become popular without all the fallout ruby is having currently.
This article starts with a slight criticisms that D isn't fully polished yet, but i don't see how an open project ever gets started by being complete and polished.
Looking at the work needed to get a language to the level that seems to be expected seems near impossible :)