Reducing CD and scaling up damage is just inappropriate for horizontally marketed game (even if scaling would be low).
This would be horizontal "ranking up" (an example):
Axii's Sun Strike:
(all ranks 120 mana cost, all ranks 15 sec CD, all ranks 520 damage)
R1 ignores targets fire resistance
R2 adds a slow effect to targets
R3 never misses the targets - (hit chance based, not players aiming miss)
(Higher rank does not include benefits of previous ranks, each rank effect only works for that particular rank.)
And then player chooses which rank to slot on his skill bar depending on what he needs for particular situation.
Player could slot R1 of Sun Strike for fire resistant mobs, or R2 for melee mobs, or R3 for evasive ghostly mobs. Depending on what ranks are available in his spell book of course (what ranks he already learned).