I really like the syntax of Agda, which is extremely flexible. It allows for arbitrary distfix operators which essentially allows for, what looks like new language features.
Although I haven't looked closely into her work I know Annika Aasa has done some work on letting programmers specify new syntax. You'll find her work on Google Scholar.
Fwiw, I think developing domain-specific languages as a library in a language with a very flexible syntax makes a lot of sense. It's sometimes referred to as an embedded domain-specific language, in case you didn't know.
