I think that if the mechanics of trading variants were introduced, the game economy would fall. This is because the game has offers towards the purchase of relics and game modes that give you relics or even assure you of certain variants (PF, Rift Battles or Story Mode) so if you could trade variants there would be no reason to buy offers, play the FP and neither the opening relics. We have to remember that this is gacha game and these are based on probability