Akaviri men/Tsaesci might, might, be enemy soldiers working in the Empire.
Given the time period, they should, anyway.
Sload, Imga and Kothringi also might make an appearance, but probably not in playable form.
I agree that there should be Akaviri.
There also need to be Imga NPC's in Summurset, subtypes of Khajiit, Falinesti is a walking tree, Vvardenfell a Temple reserve and a lot of little lore things like that.
The great thing of an MMO however is that there is no such thing as a finished game, to keep people playing there are regular updates.
Therefore, even if things are a bit wrong in the game at launch I believe that if enough people petition, for instance, to have Akaviri NPC's in Cyrod these things can be added.