Kassadin, Rift walk

Each subsequent Riftwalk within the next 20 seconds costs double the mana and deals bonus damage, stacking up to 4 times. TOTAL COST OF USING RIFTWALK N TIMES: 75 / 225 / 525 / 1125 / 2325 his riftwalk really destroys his mana why not instead make it so each subsequent riftwalk increases the next cool down within the next 5 seconds. 3 stacks. COOL DOWN: 7 / 5 / 3- 7,12,17,22/5,10,15,20/3,8,13,18(cool down reduction effects debuff) i think it would be a big change, causing kassadin to have to be more specific on when to use his rift walk and prevent him from running out of mana before he gets into a fight. also making it a bit harder to escape the clutches of his foe's. (tho the stacking damage buff on riftwalk would need to be reduced to compensate the mana he would keep inexchange)