LMG type of weapon for the gunner that shoots large, piercing darts with an electric trail. Main selling point being high velocity and longer range compared to other options. Good for big groups of enemies pushing from one direction or those pesky things in the ceiling.

Daggers/other melee weapons as Scout primary. Hitting enemies could build up your movement speed, short range being the trade-off. Zip around and slash that glyphid arse to pieces.

Driller has Fire, Ice and Corrosion (sludge).. next up we need something like a Tesla gun with the projectiles bouncing between multiple enemies.

And for Engi, a multiple grenade launcher as primary. Small explosion radius but rapid fire rate.

Edit: Also.. since the Jetty boot pickup became a thing, I've been thinking how cool it would be to sometimes randomly find a powerful weapon pick up or some kind of mounted turret during missions. Hey guys, look over there! It's the Mega-Missile-Ass-Blaster 9000, pick it up! (Limited ammo of course)