I think SMG usage has fallen off drastically because it does not scale well into endgame content. On top of buffing damage all around, I think SMGs should get a small bar that fills when dealing damage, much like glaives or support ARs. When full, you'd be able to special reload to pull another identical smg and dual wield them. Kills while dual wielding would add more time, much like Transcendence. Currently SMGs are worse than sidearms and ARs. With this change, SMG gameplay would be low/high/low/high, while sidearms would be a trusty, constant in-between.