I recommend implementing a system of diminishing returns without a cap and just balancing appropriately. With a system like that in place you are always stronger if your light or "power level" is higher and the game can still be challenging. Also do the same with actual player level.
-
To do what you propose, Bungie would have to completely separate PVE/PVP. Bungie has shown absolutely NO willingness to do such a thing. It is apparently beyond Bungie's capability.