Suggestion: don't put skills with too high cooldown.
In some games there is a mana bar, but sometimes the cooldown is so high that it takes time for the mana bar to fill up completely, it seems like the mana never runs out, so you end up not having the challenge of managing your mana usage.
In my opinion, only very powerful skills should have cooldowns to balance.
Weaker skills should not have a cooldown.
I agree that hard-hitting abilities have to be with CDs , but for the weaker ones, I don't agree.
It won't make much sense to be able to spam cast all your abilities, except 2-3 hard-hitting ones.
There can be several instant spells without CD, not a problem at all, but it would be good to have some kind of a "refresh time" like 1-2-3 sec, to prevent spamming button 1 and button 2 :D