I've seen multiple top down shooters on mobile with excellent controls. PT control implementation is actually one of the easiest to implement imho. I don't play much PT, but if I had to design a UI around it, I would:
Left joystick: player movement
- in XY coordinates
Right joystick: shooting
- you can aim without shooting if your thumb stays within a certain area of the circle joystick
- you will shoot if your thumb moves outside an area of the circle
- aiming will be "blind" as in, you will shoot in the direction that you point the joystick. shouldn't be a problem since every bullet in the game is drawn out already, so players can adjust their shots by holding down the fire joystick
Toggle Running/Walking
- option 1: dedicated button press to run
- option 2: within a certain circle area lets you walk. moving your thumb away from the center of the joystick into a larger area allows for running
Weapon Switching
- option 1: dedicated row of weapons somewhere on screen
- option 2: arrow buttons somewhere on screen to cycle through weapons
- option 3: swipe to switch weapons
Grenades/Thrown items
- tap to place grenades on screen since it's a top down shooter. just switch to it from the weapons screen
Alternative Fire
- probably the hardest to pull off. maybe a button that mimics L/R buttons on traditional game controls in upper corners of the screen, so you can use your index fingers to tap them quickly. I definitely would avoid a dedicated button in top screen corners for regular shooting though, due to very awkward hand positioning. Reserve the top corners for things you don't need to click as much
- or just tap on screen where you want to use alternative fire. Are there even guns with alt fire in PT? if yes, I'd assume they have less ammo than main guns, so just tapping where you want to shoot should be good enough. Allows players to keep the left hand on movement joystick, while dedicating right hand to switch between shooting regular guns with joystick and tapping on screen for alt fire/using grenades
Here's a quick mockup:
flip things around for left handed people or allow players to move around game controls as they see fit