Generally, weapons beyond the 1980s or so should probably diverge from real life and resemble alternate history versions of themselves with some differing shapes and details. For me this isn't necessarily a matter of "too modern" but more like "too familiar". Weird prototypes are a good place for inspiration, even modern stuff, I think the General Dynamics entry for NGSW would be fun to have. I'd also like to see more original designs (with the obvious caveat of them not being ugly like the FO4 assault rifle). I want a good mix of real world designs, original designs, and things in between.
Modders should also try weathering their modern gun mods so they don't look like they came fresh out of the factory and stick out like a sore thumb.