maybe it can be raised naturally through intense training based on base class?

1) Potions should be nontransferrable.
2) Potions should be limited. Once your total stats reaches a certain point (say 3 stats at 20 and the rest at 15, whatever that sums to - 105?), you can't get any more potions. Meaning you can get your barbarian to 20 dex, str and con, but only cha 15, int 15 and wis 15. Or if he spends them differently, if he's currently at int 5 he can get to 20s in everything else by totally foregoing int.