The 9mm AR15
was the perfect choice for the commando tier 1.
To fit commando, it's a variant of one of the most iconic assault rifles.
To fit being tier 1 and weaker than all the other assault rifles, it's technically a SMG, shooting weaker pistol rounds.
And that little stick mag is realistic to a 9mm AR15.
But eventually, with the disruptions to balance that came with the resistances and the gorefasts' head health increase, the 9mm AR15 wound up with equal damage to the 5.56 guns in game, SA80/M16/Stoner.
So now the looks don't correlate to the damage, and there's less reason to bother with SA80 over the AR15. A shame.
I'd like to see AR15 be the weakest again, by having the SA80 and M16 and maybe Stoner improved just a bit, with their base damage upped from 30 to 32. Just enough to 1shot stalker bodies with L25 Hollow Point and 1shot gorefast heads with L25 Eat Lead.
Anyways, alternate options to the AR15 could be cool, yeah, but don't hold your breath.
I don't know a lot about modding for KF2, but the impression that I get is that KF2 doesn't really support weapon mods well enough for them to be easily used enough for modders to bother making many of them. It's not like L4D2 where you can easily subscribe to a mod that makes a gun have a whole new model and animation and set of sounds, and have that mod apply while you play on any regular server you like.
You can change the textures easily enough, which is nice for things like the garish default skin of the KF2 1911, but that can't really make something a whole new gun.
If they were to officially add more tier 1 weapons to the game, that could be a way to replace the AR15 and have a whole new gun with different gameplay available.
A new menu so you could choose whether you spawn with the ar15 or the other new thing would help a lot with making that worthwhile.