Purchasing abilities and customizing the bonuses could be a way to do it....

id rather just see SE break down the walls a bit and add them to the spells themselves... Maybe make each tier do something the other tier doesn't.... so they don't become useless once you learn the new tier.

bliz 1 does the bind effect.... bliz 2 doesn't bind but does more burst damage if the enemy is bound.... that way you can utilize both tiers effectively.

Water 1 could apply a "wet" debuff that increases damage from thunder sources.