I'd like to suggest a configurable gesture, the two-finger tap.
Personally I'd like it to be set to "undo" in all of the apps I use (by default). A great app that implements this is Procreate for iPad. Makes undoing strokes fast and easy, rather than search for the undo button or keeping a hand on my computers keyboard. But I imagine that other folks might use this gesture for other actions, so allowing it to be configured would be great.
Really pleased with Astropad 2! Thanks!