I think creating an expansion that focuses on a new campaign as its primary focus is a mistake. I've never been one for campaigns in TBS games, they are too constricting. I'd much rather you focus on improving the United Planets, epsionage and other game mechanics. Having other sides people incorporated into your population mean more would also be a bonus, I'd really like to see say the Drengin in my empire get upset because I was at war with them.
Other cool things I'd like to see are:
* Ability to have Dread Lords show up in sandbox mode attacking others at random.
* Have more government options based on the type of race being played. A race with a hive mind like the Thalan wouldn't conceive of democracy and neither would a warrior race like the Drengin. For example have the Drengin player's popularity fall if he ISN'T at war with someone!
Also a race or two that liked unhabitable planets to the other races would be very interesting and having an ability to 'close' your borders to other races would be fun.
To be honest, I've never really found the races in GalCiv2 to have that much character except for the Yor (remind me of the creepy Klikiss robots from Saga of the Seven Suns), the Drengin, the Terrans and the Altarians (always liked the idea of another 'human' race out there).
Having the option where all the evil and good races group together in the same part of the map would also be good.