Trading longterm is a bad idea for the game in my opinion. Mostly because everybody would eventually have the same best characters and prize fights + rifts would become the same meta for every player. The game currently forces you to make the most of the characters you got and offers guaranteed powerful characters (like Buzkill from prize fights) which keeps the game interesting through variation.
The only exception I believe possibly would be the ability to trade natural diamonds who you have duplicates of. If a player grinds up to their 2nd diamond relic and manages to get a duplicate it is truly heartbreaking.
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