What if we made Syndra's kit based around combo'ing abilites and getting returns from it?
Make it sort of like Cassiopeia's old passive, where consecutive ability lower mana usage, maybe have it impact CD's a bit.
That way you don't buff damage, but boost up her DPS by allowing more access to spells. Maybe revert those mana buffs, in favor for this, but that would yet to be determined.
More about the mana refund/CD lowering, make it a flat amount opposed to Cassiopeia's percent, and make the next spell have a reduced cooldown.
Sound any good?
