I'd like a mod with oodles of different weapon mounts. Like point defense mounts with better accuracy and less range, point defense arrays that take lots of space but do lots of accurate damage at close range (can mount change the type targeting ability of weapons too?) Progressively heavier mounts that have reduced accuracy and increased range and damage, mounts that increase range and size (and cost) but not damage, combinations of mounts.
Basically, something like Master of Orion 2 had. I miss my heavy continuous fire automatic shield piercing extended firing arc phasors.
There is no reason in stock Space Empires IV to pick any mount other then the largest available.