Overall it looks like a great update.
The only weakness is the combo of not "practically" changing Poe (not in a way that actually changes why he is OP in arena) + being able to preview your arena opponent's lineup only forces all the Arena teams to look the same - "Add Poe or be a target". A game which has only one truly viable strategy is not a game. Sadly, this update makes the "Poe-centricity" of Arena even worse.
That said, while it takes away strategy in one area of the game, at least it adds it in others - the new events sound way cool! And sound like they will actually encourage diversification!