First off 'arena shooter' is really just another name for twitch shooter, and I do mean name because their descriptions are something different.
They could make a comeback but obviously with a small community, Halo and CoD style shooters didn't just take over because of marketing but mostly because anyone can be instantly good at them, you will find no such mercy in arena shooters.
Devs also can't come in at this late stage and half arse it, if you do it worse then a decade old game (UT2004) you are doing it severely wrong. Nexuiz, Shootmania, Serious Sam, Rise of the Triad... and some others are all recent additions but they do only a fraction of what we had eons ago, that just does not cut it.
So if someone does decide to take a crack at it UT 2004 with new ideas is your entry point, then you get inventive with business model (free version only random match making, full version for custom servers with your own setup), content creation (let users make maps, guns, modes, vehicles, mutators, ... that then get included into official rotation), also keep track of peoples records/scores and fully embrace bragging rights, then set up asymetric modes where top players go against tons of newbies, horde modes are always a good way to relax, heck let users make challenge/campaign maps of their own and things could really take off.