I feel like a smooth interface between BEAM and Julia would be particularly useful. Elixir would be a nice platform since both have lisp-y macros and 1st class AST support, so each language could pass ASTs to each other for execution. Julia could be used for heavy computational loads (which it excels at), and BEAM could handle IO, reliability, supervision, and process management (which it excels at).

An APL on BEAM would probably be terrible; since BEAM intentionally does not support arrays. Having lists all be linked lists is a deliberate choice because you have O(1) immutable copy, which is absolutely critical for passing data between procs without "sharing memory", and creating an absolutely safe concurrent system. Given the relative heavy weight of BEAM, and the featureset it provides, it's quite impressive that for I/O and routing heavy tasks its performance is within an order of magnitude of, say, C.

You could maybe bootstrap arrays as binaries, but as you start making extensive modifications to the array, you will get crazy sharding of the binary information, and performance will die.