And strict faction restrictions is something every other MMO designer is moving away from anyway. People play these games to play with their friends. In fact, in a lot of cases your friends are what keep you playing, and designers are beginning to realise that it's not in their interests to force people apart because they have irreconcilable aesthetic preferences. Since Funcom revealed The Secret World and announced that whatever faction you align yourself with in their secret war has absolutely no impact on who you can group up with to kill monsters I've been asking myself why it isn't a standard feature in every MMO with some vestige of PVP. Even WoW, the major reason that just about every new MMO even has pvp factions, just recently introduced a race that you could freely play on both Horde and Alliance. To my knowledge it hasn't destroyed pvp in WoW, so that at least puts paid to the argument that you need to be able to distinguish your enemies race by their silhouette from a mile away to have quality pvp. Though I'd hope that wouldn't be a feature in TES Online anyway - the sort of exaggerated proportions you'd need to be able to reliably tell a Redguard from a Nord from a distance when they're both in full plate armour are at odds with the franchise's traditional "realistic" style.
But I mention EQ2 specifically in the thread title because its "betrayal" system proves that you don't have to choose between, on the one hand, warring factions with distinct looks and racial identities, and on the other, giving players the freedom to play what race they want and group up with whatever other players they want. You really don't. Just let people switch, but don't make it easy. Make some kind of defection quest. Make it hard, if you like, make it a huge grind even, that's certainly the everquest way, but just implement SOME way to let people play a Dark Elf in Daggerfall if they really want to. It won't break the lore. It'll just make the world feel more real. Player characters are, after all, basically hugely powerful mercenaries whose assistance you'd be crazy to turn away because of mere bigotry. Push back release if you need to, it'll pay off, it'll bring in a ton more players who only give a damn about this game because of the license name and want the kind of freedoms they expect from an elder scrolls game. Just do it.