Ralif Redhammer
Legend
I don't see how spells are so different. They cause an effect within a range. The range is something you've already had to decide for the ranged weapons (near/close/far). So you mostly just need to decide how many things an AoE affects (which can be all in a zone or 1d6 random targets or a static number based on the diameter - the specifics are not important, only that there is a general ruling).
There's a few pells with unique effects, like Wall of Force, where you'd just have to trust the GM to go 'okay, the point of this spell is that you separate things from each other, so what do you want to separate/contain?' But, even as things exist in the base rules, many spells are murky enough that GM has to make judgement calls anyway... I don't see this as a big departure.
Sure, you could just start with "close = range up to 10', near = range up to 100', far = range up to 500'" or somesuch. But then how do you start balancing spells' areas of effect? How does lightning bolt compare to cloudkill, or burning hands to magic missile without introducing changing the power level of certain spells dramatically? If Wizards were to do it, I know I'd want a more thought-out approach than just some basic guidelines that I could've put together.