Let me preface this by saying that I appreciate that this game exists. With that out of the way let's discuss how Kabam can improve

1. A complete lack of understanding of their target audience
I really don't care what excuses Kabam has to say about this but it remains a fact that they do not know how to handle the player base whatsoever. There are a couple of reasons why this is evidently true.
Firstly, kabam cannot effectively resolve issues with their player base on forum posts and clearly have terrible PR training. I've been here for 5+ years on the forum almost every day and I have only seen one or two out of hundreds of decent responses made by a kabam member on this forum that actively gives fully transparent information with intent as to why they cannot do something or why something is not going according to plan. Where is the B2C communication that makes us feel like you're looking at us as money bags not people? You need to Admit you don't understand what you are doing with PR because we all can tell. To fix this, Actually start building those relationships with us, respond to every forum post that questions some type of problem as soon as it comes up, give shards for participating in feedback surveys, you can't improve if you don't engage with your audience and provide incentives for doing so. There should be a minimum of 5+ employees looking at the forums every workday, every work hour, commenting and bringing up every single problem that was stated up with the team and giving a response as quickly as possible. For even more effect, everytime a change is made to a champion, event, bug, put it in the in-game mail so players can easily access game altering information.
Secondly, Work on fixing the game before you add any additional content. I know this may sound stupid, but sometimes you need to take 2 steps back before you can move forward. Work on fixing every single bug and player-test all of the updates a week to a month or two before it comes out with people who play the game regularly so you can catch any bugs before it hits the fan. Go back and figure out where the problems in the community have occured and fix them ASAP. Most importantly do not, I repeat, do not ignore people with suggestions and problems. Additionally, If you need some time to go back and rethink your strategy, take the time to do so, but make sure to give generously rewarded compensation for the lack of content produced during that time to thank people for sticking around while you figure some things out. Heck even put in a quick poll to find out what players are finding the most frustrating
2. Substantially outdated rewards
This shouldn’t be a problem, but it is. In alliance quest and war, battlegrounds, incursions, and regular event quests all of the rewards for completing need to be updated at least every 6 months with the state of the game and current ability to acquire resources. Right now the rewards are from 2-3 years ago and don't at all reflect where certain players are in the game and the rewards that are being given out in other content. Where is valiant difficulty for event quests and why is thronebreaker a 6* for full exploration when it should be in completion. In incursions, zone 6 should be 8k-10k+ artifacts per room. In battlegrounds, 7 star shards should not be 16x what 6 star shards are; 5x-8x at most. Overall, the rewards are just not relevant enough to warrant anyone even trying to play the game. It takes 10-30 minutes to update this stuff. I don't understand why these specific aspects aren't being taken care of. The end game content and story content is great, keep that, but it just feels so dead in all other rewards that it’s not worth the energy or time to complete or explore anything anymore.
3. Battlegrounds
On the topic of battlegrounds it's no longer about skill, it’s about who has the most money. Seriously it's a big problem, so start matching players with similar roster power in the victory tracks. Several players have been complaining about this and it really needs to be taken care of immediately. Yes this has been talked about but It's very frustrating and annoying to be playing against someone who has a 30k power average per champion roster and you who only has a 20k power average per champion roster. Why would anyone want to keep playing if it’s unreasonably difficult and hard to reach the next tier and they keep losing over and over when they have not even completed the victory track. Here is some insight into how battlegrounds should be viewed: victory track is completion and the gladiators circuit is exploration. It should be easy to progress through the victory track, but it is up to players if they want to extend their rewards beyond vibranium 1 and take on a more difficult challenge.
On a final note: I wouldn’t be so hard on the team if everything was going smoothly and people in the game weren't complaining all the time. Where there is a problem or an error there is an opportunity to build transparent relationships with your player base.