I mean mechanicaly ...
You can "make an arena" everywhere you want.
If you mean that players can attack each other now then I don't think you can ungroup yourselves properly, players are always considered allies. For example, a cleric couldn't cast Bane on other players and have it actually do anything, and an evocation wizard wouldn't have their spells work on anyone. I could be wrong though, never played multiplayer. I am guessing that characters in stealth also show up for everyone as well.
If you mean that the whole world is a potential battleground then I agree. Shrinking the combat area does help a bit to lessen the frustration for players that don't like an extended game of hide and seek. I don't really like cramped arenas without obstacles and other things to use for cover, but some people might expect this sort of thing.
Back on topic, I would also add new companion packs to the list, although this would also be nice if it could be done with mods. Maybe new armours, but these should definitely be free for everyone.
