my ideas:

shotgun with stock
silenced smg
snub barrel 357
extended mags smg

[next ideas aren't attachments but are visual effect ideas]

left handed versions of all weps {optional}
dual-wield for all weps (including weps you can't realistically dual-wield) {optional}
upside down versions of all weps {optional}

put in your rqs in comments of this discussion