I think its safe to say that most people (and especially people interested in PvP) will be playing the Ebonheart Pact. Not only the Ebonheart Pact has the awesome warrior-Nord race from Skyrim (wanna take a guess how many people will want to play as Nords because of Dovahkin), but also the Dark Elf race that was always popular as well as Argonians which (along with the Khajiit), are the most unique-looking race. Not only that, but the Ebonheart Pact is the most warlike Alliance, and that always attracts the PvPers.
I dont want to see happen in ESO the same thing that happend with SWTOR. For those of you that never player SWTOR, the Empire faction predictably outnumbered the Republic by a HUGE margin. Bioware failed completely to recongnize the obvious problem and it became a serious issue. In ESO it will be even worse because PvP is open world, so a popular faction in many circumstances can just steamroll everyone else.
Even if there are limits to how many players can participate in battle, PvPers will still pick the Ebonheart Pact as their home because of their warlike nature. It will basically be the old World of Warcraft all over again, where true PvPers used to always pick to play as the warlike Horde while the Alliance would always lose in most Battlegrounds (thankfully its more balanced now because of many things that Blizzard had to do throughout the years to get factions balanced in PvP).
I hope Zenimax takes Faction Balance seriously. Otherwise are are going to end up with something like 5 Ebonheart for every 2 Aldmery and 1 Daggerall. That would be terrible for a game with open world PvP like ESO.
There are serveral things Zenimax could do to promote Faction balance.
I think the most obvious would be to:
1) Give substantial combat bonuses to low population Factions - This way low pop. factions can be competative.
2) Allow Faction change through a quest availiable at early levels - This way everyone can play their favorite Race and still join an under-populated Faction in order to receive the combat bonuses mentioned above.
This two things combined would surely lead to VERY balanced Factions.
I would like to hear what other people think about Faction Balance and how we can preserve it.