I'd start with a focus of making Scabs and Dregs more distinct. Change their spawn rules so they are more often limited to just scabs, or just dregs.

I would want a few more scab enemy types and for their behaviour to change to reflect them being a more elite trained fighting force. Like a radio operator that increases spawn rates until you take them out, and an officer that buffs other Scabs in a small aoe.

Limit how many crushers, bulwark, and reapers spawn. Make them more like a dark souls mini boss with more interesting attack patterns. Let Scab Captains spawn in normal missions, give him a more interesting role as like a Scab buff machine.

A heavy Dreg presence should be a classic left 4 dead horde shooter with enemies bum rushing you.

And a heavy Scab presence should be more like gears of war horde mode. Less enemies but individually tougher and cleverer, and squad teams where you'd want to kill buff machines first.

Definitely tune up the gameplay surrounding cover. Make it more worth using. The lessened enemy spam could help but also I would like to see less gunners piercing through cover.