I've been experimenting with various options for phone play alongside working out exactly how the regular PC controls interact with physics. To be fair, most keyboards already limit the number of buttons that can be held down at a time, so it's not all that different. Basically it means that there will be more pressing buttons one at a time and less holding things down.
I made a passable phone-friendly D-pad to substitute for the arrow keys, and am working on the "technique" and targeting systems together. Since playing as different types of sprites is going to be one of the main fun things you can do (either through transformation or body swapping) the control scheme has to be fairly flexible to accommodate all or at least the majority of "normal" creatures' abilities.
The default scheme will probably be more leaning towards the RPG side of action-RPG. Maybe something akin to Secret of Mana mixed with Chrono Trigger; not strictly turn-based but where there is a bit of cooldown time between attacks where you can move around to reposition yourself and select your next technique. Of course, if you simply make the cooldowns short enough it becomes full action, so the system becomes pretty flexible. The hardest part of this is figuring out what to do with sprites that have poor handling (low friction or slow turning speed) and therefore cannot be relied on to move into the positions they are aiming for, causing their attacks to go all over the place. But I guess if you make hard to control sprites that's probably more or less what you want to happen.
There's also a fairly elaborate knockback system that has to be worked out; basically knockback can be converted into flinching, staggering backwards, and falling over depending on a creature's balance stats, and repeated blows can cause their brain to take damage, reducing balance and making them easier to knock over, where they may be vulnerable to certain techniques. All these stats are optional though, basically the idea is that you can make the mechanics of a creature either simple or complex depending on how much effort you want to put into its design. Sort of like Dwarf Fortress and its various immunity tokens, except that here being immune to everything is the default state.