I think it will end up being a million legendaries in the top decks. I mean, Poe for instance runs a lot of legendaries, same with Vader. I belive that those decks will dominate, whoever puts most money into the game will win, just like any other ccg ever made in the world. I hope it's going to be different, but I am already so underwhelmed with the booster format that I am expecting the worst.
This game is really good, and I would recommend it to anyone, but I feel like it should definatly have been an LCG.
A complete deck is 30 cards + 1 battlefield + between 2 and 4 characters. So, a maximum deck size is 35 cards.
Doing the math and building solely to put the maximum number of legendaries in one deck, Villains can get 12 legendaries max and heroes can get 11 legendaries max. At absolute maximum, you can only get up to about a third of the deck currently, obviously with more sets this will increase (it probably won't be until set 3 that you can play a deck of exclusively legendaries).
This being said, currently, playing every available legendary doesn't necessarily synergize all that well together.