Elder Scrolls Online has something similar to this.
Clarity:
To ensure both factions maintain their own identity, these items would either be made available for purchase in the shop for a pretty high $ price, or acquirable by achieving some high difficulty feat. Additionally, players who use races outside of their native faction would suffer a de-buff of some kind in PvP. aka if player wants to play a Sacred Order Demon, they will need to be willing to sacrifice a decent amount of time, money, and/or character effectiveness in order to do so.
What it is:
An any race, any faction upgrade would allow the user to select any race they wish to play as, regardless of the faction they play.
Meanwhile, a faction swap token would be a one-time usable item that can be used by a player to swap to the opposing faction.
Why would someone want this:
Roleplay; Having a character stained with treason could make for some cool roleplay among the RP community.
Gameplay; Groups of friends may like different races/classes from different factions. Allowing these races to exist on any faction, even with a huge hurdle in the way, would help these groups.
It is fine what you want to raise, as it has been implemented in other games, what I do not know if it will affect the essence of both main factions as history, etc.. But what I would do is that with a transfiguration token (With many character models in the store to buy with real money, with enough gold from the game, or you get it when you defeat strong enemies) to use that as a method to make rolplay or use magical disguises or with potions and you transform into races of the opposing faction, without powers, only the appearance. And as a second option implement your idea in servers that would predominate the roleplay.