Honestly I think there'd be something to enjoy for all of the remaining factions, so I don't have a strong preference. I really enjoy all 3 campaigns, and aside from the occasional massive battle, that's pretty much all I do with the game so far.
The only multiplayer I tried was in beta, and Chaos was my favorite there, so I think I'd enjoy that a lot. They have really cool ships. I'd hope for an Ordinance fix prior to a Chaos campaign, though. Plus I think it'd be the most interesting story of the remaining factions. Thing is, if they did do Chaos, I'd prefer to play through the fall of Cadia than the follow-up story arc that the main game follows. i know that almost certainly won't happen, but it'd be pretty cool, and it's a pretty epic story.
Orks would be fun for both comedy and (lower case) chaos factor, plus the very different play style and neat upgrades.
Eldar seems to be the most diverse option, since there'd be 3 factions to include in the campaign. On a personal note, I tend to just find a gas cloud or asteroid field & hold position with all my ships in there for most/all of the match & kill everything on its way in. With Eldar being forced to move to maintain holofields, I'd have to mix it up a bit, which would be interesting.
Tau would also be interesting for having a unique play style and interesting upgrades, though it might get a little same-y after awhile. Still, it'd be nice to have another "good guy" faction, just for variety's sake. I do think this is the most likely option right now, since they're not in single player in any other capacity. Plus that visual bug really does seem like a very strong indicator.