I wouldn't be surprised if Bethesda would throw in humans-half-mutated-into-fish or something similar.
Normally I would welcome any new mutant varieties to the Fallout universe as long as they don't fit a trope. (Ghouls in 1 and 2 despite looking like zombies in general being intelligent and civilized, Super Mutants despite looking like orcs also not all warlike, even the dumb ones weren't necessarily evil)
But I fear that any human-fish mutations would fit the Deep Ones stereotype (xenophobic, cultists) instead of a fully realized and working concept.
Wouldn't it be nice that when you encounter such beings and manage to get them to drop their cautiousness and suspicion of you that they turn out to be some of the most friendliest and civilized beings in the area who are even disgusted by the idea of murder, lying, and theft?
(heck I might even make them realists who fully realize that they are mutations and not have any belief in some kind of fish god)