I do have a few weapon ideas, but the two that would probably add the most variety to the game would be
1) A gun with a tighter spread and more range (probably a Sniper)
2) A slower but stronger weapon (maybe an RPG)
A Sniper would make fighting some enemies, like the wall squares and the bomb guys easier to combat, while opening the door for more ranger focused builds
An RPG would make later levels with stronger enemies easier, while still forcing the players to rely on good movement and good upgrades
Hopefully these suggestions are good, and I'm interested to see the future development of this game!