I would be happy with a simple system.

Say, any Tier 1 spell can combo into any Tier 2 spell, providing different debuffs/effects depending on the combos.

So maybe fire >> fira would be for maximum fire damage and no debuff on the mob.

But fire >> blizzara would be low damage with a powerful paralyze debuff on the target.

Or fire >> thundara would be moderate damage with a short stun effect on the target.

Something simple enough for everyone to understand, but complex enough that you don't get bored with thunder combo's all the time.