The thing that Brawl Stars has over most of the games on this list is that sense of polish and balance you only get from major developers. On mobile, though, no one gets more major than Rovio.
The maker of Angry Birds has its own casual team-based arena shooter in Battle Bay. It's a brilliant 5v5 shooter with intuitive controls and brilliant 3D presentation, and its aquatic setting makes it feel quite different to anything else.
The .io genre is quite different to what Brawl Stars is doing, but Munchkin.io brings it much closer. It marries the .io concept of all vs all survival of the fittest arena combat with cute hero characters and a variety of melee and ranged attacks.
If the comparisons with Brawl Stars weren't already apparent, it also throws in a Battle Royale mode. It's not as slick, but its expansive map and action RPG combat might be more to some people's liking than Supercell's game.
Mayhem has a similar premise to the main Brawl Stars mode, in that it's a 3v3 arena shooter with both teams vying for control of the map. Here you control a central capture point rather than scrapping over gems.
But the main difference with Mayhem is its 2D side-on perspective, which makes it feel more like an action-platformer. It's a pacier, more acrobatic experience as a result.
Here's another cute take on Battle Royale that might appeal to players who have been left underwhelmed by Brawl Stars's Showdown mode.
Guns Royale is an enjoyable top-down twin stick shooter, but with more pronounced BR mechanics like looting and a bigger map. It's also got a Minecraft-like blocky aesthetic, if you like that sort of thing.