Well if a melee attachment system were in place, I could go for some taser and poison attachments. Perhaps poison as a poison reservoir with a large storage and a use-activated applicator/it slowly leaks onto the striking surface would be more thematically sound if limited-use attachments were difficult.
Hrmm, if limited-use attachments are not difficult, perhaps railguns could have rail attachments that are matched to a massive disability in railguns, so, maybe they might have +50 damage and the railgun has -50 damage… After firing the railgun a few times the rails would wear out and need to be replaced.
More melee attachments:
A hand-guard to add parry.
Fine weights to make it better for throwing.
Simple weights to add bash.
Barbs to add pierce.
Blade extension to add cut.
A strap and chain to make it disarm into your worn items.
An air-catcher so that it makes a cool noise as it is swung, so it makes noise but maybe adds a limited morale boost.
You could have attachments for clothing too. Sequins to make it fancy, lead strips for environmental, some extra straps could reduce the encumbrance of loose items but make them more arduous to actually use, clip-on pockets for capacity…