⎕FMT is, in a word, venerable.
It is still, I think, indispensable, but it was designed in the days of character-based reporting, and in the age of HTML, perhaps it is time for a new version, ⎕FMTX or some such.
A couple of things off the top of my head:
1. The field width should not be required.
2. repetition (r) and alpha (A) are probably obsolete
2. Applied to a numeric scalar, the result is char vector, to a numeric vector a vector of char vectors, to a matrix a matrix of char vecs, etc.
It's not clear to me if the current specification should be used as starting point, or if it would be better to start from scratch.