What if Clarity reduced (non-R) cooldowns by 35% ?

If it's going to reduce cooldowns, that would immediately become the main function of the skill. Mana restoration would be the secondary effect. Given that mana regen is currently Clarity's purpose, effectively deleting Clarity's main role.

Second, it would immediately become used as a mid/late-game version of ignite. On anyone with any kind of length on their cooldowns, I'd use it. Tanks, Assassins, burst mages; everyone. It would surpass the potential damage very early on because spells scale so much better than ignite both in damage and in utility. The usefulness for some champions would be absurd.

A flat reduction on cooldown would have to be given a new summoner spell of its own, and it would likely have to be significantly weaker than your proposal. Given that it's really just directly competing with other summoner spells in terms of increasing burst or CC, it's probably not a good thing for the game. There might be a way for it to function, but it would need a lot of thought to go into it, as well as rebalances of other summoner spells.