To not interrupt with weapon related skills provided in the skill tree yet provide greater customization, weapon mods should be reserved for such things as scopes, sliencer/muzzel, extended mags, better coolant (to keep from overheating as easily), different rounds (depending on the type of gun to affect usage but not actual power), and possibly even attachments. In short, actual physical modifications to the weapon. If there are mods that greatly increase handling, accuracy, fire rate, or power then that would defeat the purpose of buying different tier weapons at all. The weapon mods should only be slight tweaks to performance but also change the weapon visually. I know some think that is stupid but slight tweaks mean a lot to some. Plus, several slight tweaks together may add to a very favorable result.
To unlock weapon mods should require skills in the skill tree and sometimes (depending upon the mod) require more output from the dropsuit being used.
Allowing more diverse weapons alone would solve the want for more diverse weapons in the short term but I think it would increase market activity to have an overall set of diverse mods to customize the current list of weapons.