The most productive remapping so far which I made is to globally remap (via xkb) Left Alt + HJKL to respective arrow keys. Works in all GUI applications which rely on arrow keys for navigation, plus standard Ctrl + Shift + Left Alt + H (Left Arrow) works to select previous word in text fields (e.g. in browser).
Next one is to remap Left Ctrl to Left Shift and to press it with upper side of the palm just below pinky (not sure how this part of the hand is called correctly) instead of utilizing finger for that. Works best on mechanical keyboards without bevels.
Also I find quite useful to remap keys 5-0 to Right Shift + Q (5), W (6), E (7), R (8), D (9), F (0). I find these keys are easier to reach and type without a mistake instead of using standard 5-0 keys which you should stretch your fingers to type. For example, to type "()" I use Left Ctrl (remapped to Shift) + Right Shift + D, F which is easier to type without fingers leaving home row.