J's ideas are terrific. Dyalog is actively incorporating the ideas in J back into APL. J's choice to use ASCII characters instead of Unicode is a historically understandable design decision that I feel should no longer constrain the APL community. Otherwise, Dyalog APL and J are my two favorite APLs.

If I can't write in APL, I would probably write it in Scheme. I haven't found anything yet that wasn't quite addressable in APL, and the one that was (writing compilers) I figured out how to do, obviously.

However, if you want to create a DSL or otherwise play with notational elements in a DSL style for which macros are the "right choice" then APL is unlikely to be your friend, because I have yet to find a way to create good macro systems on top of APL. Scheme simply wins here. However, vocabulary based DSLs (combinatory DSLs or the like) are very well suited to APL.

Outside of these, I'm actively exploring the idea of theorem proving based programming, and currently things like Agda, Idris, ACL2, or Isabelle are going to be better for this right now. Not inherently, but because of tool support. I hope to change this in the future.