a 420 is about 110 gallons right?
If you want some semi-aggressive fish (not including some clowns) you should get one or two large peaceful fish to control the tank (when my old naso died, the tank started to kill each other so we 2 kinds of tangs and a rabbit fish). For example you could get a
yellow tang. Others you could have is of course a pair of clowns, I would get a lawnmower or some kind of blenny. also get some schooling fish.
I'll have to upgrade my 125 gal. to something in between 200-300 later probably in 2 years.