+1 for exhaustion being in the game (if only because Larian could use it to encourage our characters to long rest that first time, when we might otherwise be too scared to).
+1 for using the implementation of it from the One D&D playtest (cumulative -1 penalty to everything for each level of exhaustion). I'd also argue for -2 foot movement speed per level.
However, I think there should be something that allows the restoration of more than 1 level per night besides Greater Restoration (5th level spell = not accessible until 9th player level). With this weaker One D&D form of exhaustion, perhaps lesser restoration could restore a level? Or if you upcast it to 3rd level, it can? This would allow Larian to play more around with monsters giving out [multiple levels of] exhaustion during combats, without it being as absolutely punishing as getting multiple levels of exhaustion is in current 5e.
