Changing the additional slot gain from every 2 levels to every 3 would help a bit. Limiting memory to +1 or +2 on a couple of item slots (probably like helms and amulets only) would help as well. I think the larger problem is really the lack of requirements for various powerful abilities, as well as actually not very many skills in general. When there's 20-30 skills in every tree you'll sure be scrounging for all the memory you can get.
I think the solution is pretty simple for ability requirements: a memory cost penalty for low ability points in a skill. So if something requires 3 Necromancy, if you have 0 Necromancy it will cost 4 memory. If you have 2 Necromancy, it will cost 2 memory. This creates a dynamic where you can splurge on an ability out of your expertise for a certain situation, but you'll be sacrificing other skills to do so.
