1 - Do something to address Arena grind/bottleneck in player progression. There have been dozens of ideas offered up already. Pick one and implement it as the status quo is unreasonable. Here's another one for you:  limit which champions can be used in different tiers in order to stop fully built legendary teams appearing in lower tiers.

2 - Let us run our own heroes against each other in order to work out our synergies and arena teams. No energy or reward needed, just a button in the sparring ground called "practice", let us be able to set up 2 teams up to 4v4 and have them battle, with the option of controlling both sides or, of course, auto. This should be straightfoward to implement as it's not really adding any new mechanics to the game, just giving players an extra option, and something to do when they have run out of energy.

3 - Actually reward people for spending money in this game rather than frustrate them. At the end of each calendar month, give people who have spent money on your game a choice between 3 epic or legendary champions that they don't yet have. 

This is only potentially 12 champions per year, costs you nothing, and with the ever deepening champion pool has zero impact to your business model. It may even encourage players to spend a bit if they know they are going to get a guaranteed champion that they don't already own.

4 - Review your pricing model. It's astonishingly greedy, and all you are doing is selling bits of imaginary data. You can afford to be more generous than at present. Case in point, I can buy the entirety of a recent AAA release such as Outriders for the cost of what you seem to think a few books are worth that provide marginal stat increases to individual champions.

5 - Implement some sort of upgrade/customisation path for gear in terms of stars and rarity, or even switch out a substat or two. It provides another path for players to grind the gear they want, and worthwhile drops are rare enough that it wont affect the time for players to gear their characters up. I'm sure people would rather be making tactical decisions on a pool of gear they own and can adjust rather than mindlessly grinding for speed boots and complaining about the arena, and this would therefore more effectively increase the longevity for players.

6 - Display the pity summon counters.

7 - Look into integrating Discord to replace the in game chat function if possible?

8 - Graduate the items being offered on the marketplace in line with the players account progression and remove/increase the cap on blue shards being available by the marketplace. It might also be interesting to be able to spend gems to boost the quality of items being offered on the marketplace for 24hrs

9 - Add a function in the champion screen that allows for players to create and recommend builds for different champions in game, that other players can up/downvote. This will help newer players to understand what to aim for in order to develop a specific champion.

10 - I would really like to create sections/buckets in my roster that I can drag and drop champions into. Even with the new symbols, champion organisation is still quite messy, and im sure players would like to be able to create groupings for factions, food, their clan boss team, dungeon teams etc.