This is just an idea, but I don't think they will stray from the main weapon types (LMG, Rocket Launcher, Mini-Gun?), though they may give us the option to receive opponents guns (like if you go through a stronghold). With this is mind, I doubt they'd make an alien gun shoot bullets. That'd be boring. The weapons may be based off of models from common weapon types and just shoot projectiles other than bullets or have some awesome animation while doing it. I hope Gjallahorn, an exotic rocket launcher, does something completely awesome, not just shoot a single rocket. Bungie said exotic weapons would stand apart from the others, so I don't want a rocket launcher with a cool name if it only shoots one normal rocket. But if I really wanted a heavy weapon, it would definitely be a mini-gun.