Random crazy ideas about the MegaBuster:
When first discovered, it could only have one mode available (a small damage 3-shot plasma burst). It would take cells to reload. I also think it would be interesting if this mode could be charged (perhaps by a regular reload, prompted, when already full?): the first charge takes a certain amount of time, changes the attack to a more damaging one-projectile plasma attack, and burns half the clip. The second charge takes even longer, changes the attack to an even-more damaging radius 1 plasma attack, and burns the whole clip. Kinda like the Beretta in reverse, plus the time to charge.
When enough of certain enemies are killed while wielding the MegaBuster, it "learns" new modes based on those enemies, most likely toned-down versions of their attacks. For example, killing the Cyberdemon might give you a "rocket" mode with a damage of 3d3 and blast radius of two. Killing a Shambler (or two) would give some lesser-powered version of their lightning attack. Killing the lava elemental would give a mode similar to the Napalm Launcher. The clip size and required ammo type would be different for each mode.
Lastly, the MegaBuster would allow the player to change modes via Alt-reload at a small time cost (again, similar to the Beretta).
I don't know if any of this is even possible or balanced, but from a "flavor" standpoint, I think it would be interesting.