Personally, if the expansion did nothing else but give a solution for population imbalances I'd be happy.
Ah fuck it... here's a list of good ideas I think they should implement just to fix their fucked up game.
Population imbalance solution:
Allow any race to start in any start location/faction. This doesn't go against WC mythos.
But provide incentives for players to start in their native locations, and disincentives for players rolling none native faction races (not crippling but annoying; such as a rep increase been only 80% of normal).
Also, allow players to learn the languages of different races; including opposing faction races and what not. Wouldn't be particularly easy task, but still doable.
Similarly, allow players to convert, but not without a large deal of difficulty; say having to drop all current faction rep down to below neutral and having to work rep on the opposing faction from a neutral town such as gadgetzan or maybe a new refuge town built for such a purpose.
Also, with regards to BG; they could really fix things up and spruce it up a good amount...
Right now, the system has too much emphasis on grinding and not enough on just enjoying the actual PVPing (when it's good, it's really pretty damn good); things like population imbalances, skill imbalances etc, can be resolved with a cross server BG experience. People on the same server/faction are queued together and then matched against another group against any faction/server of an equal skill (which is judged based on how you do against other groups; your group performance is ranked for each game then your personal performance/skill level is made up of an average of all the ranks of the groups you've played with, with more recent matches given more weighting...
when it comes to matching the groups, they'd be done on the basis of taking the sum of the personal levels in the group and matching it against a similarly scored team).
At the very least, that would create less delays (especially when there can be alliance vs alliance or horde vs horde games) as well as providing more balanced games.
Ideally, they'd also create more maps for each game time, with different maps for a different size in groups (so you can have 5 man WSG runs going or 10per team AB runs or 20 per team).