Side thought, in regard to difficulty grade, area advancement and confusing the player...
What if they used monsters and creatures that all adhered to the actual game rules - and for custom monsters, they build them using the rules and calculating their CR correctly etc., but then, whenever you were actively in combat, it would have all of the enemies present with their ratings, and it could use the game's own rules to give you a peek at the projected danger grade of the encounter you're currently in - a little signifier on the UI somewhere, related to the combat elements. Something subtle but visible.
So, you get into an encounter with a group of creatures, and you're not sure if you're in an over-levelled area or not... but you can look and see off to one side on your UI that it's calculating the current collective of enemies in the initiative order as a 'Medium" encounter for the allies in the encounter. It's nota precise thing, and CR definitely can give false impression in some cases, but it's a guide line, or a rule of thumb that would be simple and helpful to the player, without being confusing.
The enemies would have to play by the same rules as us for building and stats for that to work, though.
