Thanks for your comments, I understand it's difficult to imagine something completely different from WoW and that my suggestion for factions "joinable" independent from races would be very impactful for the game lore. However, I still think it could be a nice idea. In my opinion, the unbalance between factions not necesary will be managed with unique races in each faction. Because even with this concept, we all know in WoW there are far more population in the horde in most realms, and most of the professional raid guilds are horde too.
If the race selection is independent from the faction and you can opt to join a faction, stay neutral, and even abandon a faction an join the opposite, you can balance the racial traits in the PVE and PVP scene. To keep the balance of population you can offer increased XP and gold rewards, faster mats gathering, etc. Also the pardon fee to pay if you want to change your faction may be significantly different if the faction population is not balanced, so you can offer a cheap fee to enter a faction less populated (because they are recruiting) and a very expensive fee to enter the most populated faction, also losing access to those nice bonus XP and rewards I mentioned.
Of course my idea is not defined but the fact is that we don't have to think exactly as other games.