Let's steal some design concepts from the classes of Deep Rock Galactic.
From the gunner, we can steal the Anchor Line Deployment System, a rather nasty harpoon gun and tough cable that can be used either to tie the enemy down, or to create a mech-scale zipline. Attach several to a landed dropship to create Fun on liftoff. Gunner has guns, what do you expect?
From the Scout, let's steal the Phereomone Canister, for getting the wildlife to do your fighting for you, and the flare gun, because we sure can't steal the grapling hook that lets you attach and zip around like a gremlin. Scout's not good for ideas either. Maybe turn the Inhibitor Field generator into a 'annoy PPC users' weapon that treats any target within the field as at minimum range?
Driller, on the other hand can give us several things. Mech-sized satchel charges for demolitions? Now you can cause machine gun ammo explosions on anyone you can attach this to, even if they don't have machine gun ammo-just be careful, it has literally 0 armor and any damage can cause an ammo detonation. The Sludge Pump, for when you want to spray thick sticky acidic goo all over your enemy. The Colette Wave Cooker, a long-range microwave-based flamer, which...doesn't cause heating to armored units, heats up itself way faster than you are comfortable with, and might be a minor warcrime if used on civilians. Just use it for defoliating, please!
Finally, we have the Engineer. Say hello to Star-League era polycrete foam projectors. Do you want to build UP the terrain, instead of knocking it down? Do you want to build a shelf a mech can stand on? Spray fast-setting Polycrete and stand back for 12 seconds as it sets as hard as rock, but is still as light as pumice! Use it to seal wall breaches, to block passes, or to simply create platforms on flat ground to stand upon. And with our new shock-absorbing compound, it can even reduce damage caused by falling onto it! Or mix in xenofauna repellents to funnel native animals away from your construction sites. Or just...overclock the inertia inhibitors and spray rapidly hardening construction materials onto your foes, you might actually disable them in this way. And moving on, we have other options. Use a starship-disassembling breach cutter to slice off limbs, if you can get the unweildy monster on-target. Use a swarm of flying wiring robots to attack your foes! Deploy ECM lure that generate ghost targets!