Not gonna happen for several reasons. Compromised accounts happen and champs get sold. At least you'd have your 5* champs. Would also make it easier to get 6*, and Kabam doesn't want to make that too easy. Trading will NEVER happen as there would be many chances to abuse the system.
Imagine you have a friend that is just leaving the game and you give him an FlixDD and he gives you his Blade lol. Or that you want to clear certain content and then you trade champs with someone and you give them back when you are done.
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That would be way too game breaking, tbh.