Awesome game don't stop adding new features because this may become the most popular mobile game.
I came up with a pretty amazing idea, its very unique and I think you'll like it.
-add a 5 Vs 5 (any suitable number). This mode is a match between players. Here comes the awesome part. In this mode, the usual gear used for killing titans will not be used but the gear which Kenny ackerman and his military police team use in season 3 will be used. You know the one with dual guns.
Now are tips to make the game the best of the best.
-add some attack on titan music. Make it so that the music intensifies when titans get nearer.
-focus on graphics and even the tiniest details. For example the loading screen and the random behaviour of the characters. Improve the character design.
-if you want people to keep playing your game, then you have to add things that players love. For example everyone loves eren's new look in season four. So add that skin(be careful to make it as good-looking as possible). As a cherry on the top, make it so that players have to finish a few missions/achievements to get that skin. If you add this feature players will be playing non-stop.
-everyone loves good graphics. Work on the realistic look of the grass outside the walls and the look of the trees. Make random destroyed villages. Improve the look of the sky. Add sounds of birds and the wind. Improve the look of the buildings and the ground the character walks on.
-add scary roars or screams for the titans
-when the character lands on the ground from a very very high place. Make the character roll. Animation and smooth/realistic actions should be added.
-make it easy to invite friends and call them to a squad with four or five people. Make this part like fortnite or pubg. I am talking about the part where the squad is standing in a group until all the players of the squad click on the button called "READY". Here I have a very cool idea. Whenever jean and eren are in one squad they should stare angrily at each other or start locking hands and start wrestling.(take note this only happens while the squad is getting ready in the lobby) if mikasa is also there in the squad then she should try to stop the fight by holding eren's hand. Trust me, players will love these tiny details.
-add models of all characters from season 4.
-add a special mode where players are in dorms and there are no titans around. Just like a place for players to relax and stuff. Add shops there so that players can go and buy accessories like a cooler looking blade or something, headphones and whatnot.
I really love this game and I'm sure this will become something big if you follow all these points. You are awesome don't forget that.