There are a couple of things I wanted to do when I set to work on modifying the spell rules for B/X. First, I wanted to allow magic-users and clerics to dabble in each others’ domains, and to let any class dabble in both magical pools if they had the will to do so.
Second, I wanted to erase spell level requirements wherever possible. That is, a magic-user should be able to cast any spell he can get his hands on. It makes even low-level wizards weird and fearsome – you never know what kind of magic they will be able to bring to bear.
The same is true for clerics. A low-level priest who has obtained a dreadful high-level spell is now a terrible threat.
I feel like these things drive adventures all on their own.