I believe this mod is amazing with it's simplicity while still having 3d guns that look, sound, and play nicely. Other gun mods tend to add too many guns instead of modularity which I think is better for a sandbox game. The main feature that I'd suggest for future updates are more attachment options like extended mags, lasers, flashlights, muzzle brakes, compensators, and night vision scopes. Other than those more stocks, grips, and barrel length options could do a lot for the different visual combinations.

I second flashlights. Especially in the case of modpacks that use this mod for apocalypse additions where lighting may be set super low in closed areas.

Glass tends to have odd lighting interactions, so perhaps utilizing that mechanic you could create a more "modern" type of light by cancelling out the lighting. This would work well for aiming in dark areas, but may have shortcomings when considering an infinite range flashlight just like how looking at a glass entity.

Learning shader/dynamic lighting tech is also hard. I'm sure plenty of shader authors/technicians would love the challenge this provides. SEUS and Rethinking Voxels are two projects that utilize light and shadow propagation. Unfortunately both of these things require shader support like Optifine, Oculus, or Iris.