Formats on Arena have serious problems. That's because, since a lot of players play every day an enormous number of matches, metas revolve very quickly. An online game should have ban, rebalancing, and sban every month at least, but every two weeks possibly. On the contrary, now formats are completely lost and left to themselves.

I will now talk about the arena-only formats. Also, Standard has problems, but you already know (you should have acted sooner, also in this case).

Historic needs to be reformed.
The game is getting more and more boring every day. In most of the matches, it seems to see opponents play a solitaire game. You don't have any possibilities of interaction with some of the decks now present in the format.
Especially in Bo1, the most played format in your app, Magic is becoming very boring. Reanimator persist combo is very annoying. Now it consistently closes during the second turn, and it is very difficult (and boring, since the matches are all the same) to play against it, since you should always have the exact answer in the first turn.

Eldrazi is simply horrible, since I don't understand why only this deck can have such synergies that completely prevent opponents from playing. Destroying opponents' lands without the possibility of having a land back and without any drawback for the Eldrazi player is horrible in a format like Historic, also because every other strong way to interact with opponent lands is banned. And also, Eldrazi is the only deck that has two ramp lands specifically made for the deck. Why should any other deck have the same possibility?

That's why the format is becoming a solitaire card game, and why playing Magic: The Gathering has never been so boring and even frustrating, to the point that I prefer playing other games, and it happens frequently that I shut the app, passing to other, more passionate games, such as a real solitaire. Game experience is no longer the same as before.

Timeless is simply unplayable, since it needs way more cards to be even considered as a nice and interesting format. You have already added some cards, but this is not sufficient. And also in this case, you have to rebalance the format faster and to act with haste when a problematic deck presents, because the problems in this format emerge very fast, and the format is not old and rich enough to reduce the influence of bad cards.

In a simple sentence, you have to care for the formats. Now we, as a community of players, feel completely lost.