Ability to run, I already suggested. I like 3runs idea of stamina to sprint. But while you're waiting for the next wave, there should be no limit.

I don't think, that the revolver should be balanced with the other weapons: harder leves, bigger weapons is the device of that game! But you're right that the later weapons are not that good balanced - you always use the mg - never use the later coming shotgun (much to slow reload time...)