Personally I could see NIN adopt a DoT juggling/poison stacking playstyle (similar to ARR SMN) to replace its combo abilities. The only reason Gust Slash/Spinning Edge exist in the rotation is to act as filler. You could easily just remove the combo effects from their hotbar, add Mutilate back in on Gust Slash, then put the slashing debuff back on Aeolian Edge while balancing out the numbers and leaving the positional requirements as is. By staggering out the DoT/debuff/buff timers you could encourage a similar behaviour to combos where your DPS dips and spikes in short intervals while freeing the player to make decisions about what to press next at any given moment instead of "1-2-what do I do?" But I understand that kind of gameplay is also harder for new players to catch on to as the free-form nature of it makes it just as easy to screw up when you're unaware of how your kit works.