Timetwister is banned in every format except Vintage and Commander for a reason. It’s extremely powerful. For only three mana, you and your opponents shuffle your hand and graveyard back into your library and each draw seven new cards. While it’s a symmetrical effect on paper, depending on the state of the game, it may not be. Putting that Timetwister effect into Modern is quite dangerous. Sure, Echo of Eons costs six mana (4UU) to cast, but it also has a flashback cost of only three (2U). That means as soon as Echo of Eons hits your graveyard, you have a functional Timetwister at your disposal.
Of course, because it is six mana to cast normally, it doesn’t appear overpowered. In fact, at six mana, the effect is probably almost fair, since all players benefit. But it’s not hard to toss this card into the graveyard early. There are decks more than happy to toss it in the bin and cast it for its flashback cost as early as turn two - perhaps, even turn one!
The sentiment among the player community is that it’s more likely that Echo of Eons will be more broken in Legacy. Some people are even expecting it to be banned. Then again, as it plays out, it will be interesting to see if it’s as powerful as it appears on paper. After all, Day’s Undoing - another card similar to Timetwister - never took off, but it also ended your turn if you cast it on your turn. Even so, Day’s Undoing has found homes in Modern and Legacy, although not in high level decks. Echo of Eons may prove to be better, but only time will tell.
The one deck that has tried out Echo of Eons with some success is Ad Nauseum Tendrils combo in Legacy. It's typically just one copy in the sideboard as a target for Burning Wish - also the way the deck often gets Tendrils of Agony into its hand. It's not a key component of the deck, but it's certainly a good card to have in a pinch.
Do you think Echo of Eons will break Modern, or more likely Legacy? If so, will it end up banned in one or more formats?