There's not cooldown period on mailing to characters on the same account so creating a complete new system for just sending over a few legendairies to your other character seems pointless if you ask me.
I managed to get a lot of legendaries with ingame money over the past few years (30+). It would be dreadful if those legendaries became untradable since they where aquired with the intention that they could be sold on marketplace or for a friend to borrow. I'm sure I'm not the only one who has the same problem. However, I do not mind buying new Joker boxes, knowing that they would not be tradable. So the bottom line is that I think old legendaries should remain as they are, new once can be character/account bound.
The old Innova (4game) already had a simular thing in place: most JB legendaries where character and just a few where tradable. This led to overpriced ingame marketplace prices tho.