During the time of The Elder Scrolls Online, the Imperials serve under the rule of the Tharn family in Cyrodiil, who has an alliance with the great necromancer Mannimarco. Said necromancer do in turn serve Molag Bal, who plot to drag all of Tamriel into his plane of Oblivion.
At release, Imperials are not playable. As for the future, one can only guess. There are many possible reasons to why Imperials may or may not become a playable race, including but not limited to desertion. As their home province is the seat of (to my knowledge, all) PvP content, it is likely that any playable Imperial might have to choose their allegiance to either the Pact, Covenant or Dominion.