I suspect the Vasari's enemies will be introduced - maybe as a playable faction.
I don't think implementing another race will be part of any "mini"-expansion as it would be way too much work, especially when it comes to balancing.
3 races are perfect for balancing RTS games, even blizzard knows that 
1st expansion is about defenses
2nd expansion is about trade, economy and diplomacy
so the 3rd expansion would be either more offensive options like more ships or automated attack-drones, mines, siege-weapons etc. or even better -> more tech and end-game weapons so that you still have something to research and gain from in large maps.
ah and when it comes to engine-upgrades multi-core support would be awesome.
right now it's impossible to wage war on really large maps with 8 players on it even with the most top-end hardware.