I did consider having somr sort of collision like a river to break the arena up, but ended up not implementing it - i agree that its probably better without annoying collisions. If you hit something, its an enemy. I did consider having the fire click as a 'base' weapon, then the others as temporary powerups, though ended up going in a different direction. I definitely appreciate that the upgrade system isnt for everyone.
Admittedly I didnt really think too much about each weapon as the "only" weapon while making them, but then the upgrades made that a possibility. The lightning was originally meant to target multiple random targets but for some reason all bolts targeted a single creature so i tweaked its functionality. I think it balances the power it has by removing the control, but its definitely one thats harder to make use of as your only weapon.