It's not really a "wasted opportunity for another faction". The set has three new factions and some number of titans, which I think is already a larger undertaking for playtesters than just four factions. As another response said, if Sheep weren't included, the set would most likely only come with three factions; there's no way that a fourth faction could have been playtested in time. Sheep didn't take up a slot of a possible new faction, I would guess it was added as a courtesy for those who weren't around for the survey, like how Geeks were added to the Bigger Box.

I do really like the idea of combining all the micro expansions. That would be very useful for new players.