I think the best way to improve the popularity and damage of sidearms is to have the option to dual-wield. This would just look bad ass and give sidearms some much needed punch. You can even add some duster class items to celebrate the change.
Titan: Dual holsters in the titan mark slot
Hunter: Duster like cloak
Warlock: Duster like robe
Who doesn't like a western theme?
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I'd like this a heavy weapon alternative. Scout, shotgun, dual pistols. The Lobby Scene from The Matrix comes to mind.