If you forced me to implement something without doing any research, I’d suggest:

First, (iOS perspective but it’s not that different than Android) to convert the suggestions row to a row of function keys anytime Shift was held down. No annoying delays or extra taps waiting for the menu pop-up. So now you can use two fingers to cut copy paste undo instead of using several seconds and a popup that can appear anywhere onscreen. Next, arrow keys. Heck, use the same Shift hack and, when Shifted, replace the space bar and the dead useless area below it with a big inverted T. Our phones have gotten much bigger and yet especially on iOS the keyboard hasn’t grown a pixel since what, iOS 5? Case in point: no number row still for Apple, even as an option. Wtf.

iOS already has three-finger pinch to copy/paste, and three-finger swipe to undo/redo. But I find that these gestures are too unreliable for regular use, the gestures almost always activate some other click target plus my other fingers are busy holding the phone.