Balancing around mana cost doesn't work when you have infinite fucking mana, which has been the case in every diablo so far and dragon's dogma. And if the animation is twice as long because it does twice the damage, all you've done is create an inferior skill nobody should ever use. To say nothing of things like buff spells, transformations and so forth. You can't balance a skill that makes you temporarily invincible with anything except a cooldown.
Not true. Repel can negate all damage and has no cooldown on Dark Souls 2.
As for Dragon's Dogma, is a amazing example of game without cooldowns. Ricoshet seeker/hunter can be the highest DPS skill in the game vs living armor in CQB and useless VS lightning dragons at range.
You can balance skills that give immortality by for eg, make it costs gold and make you bankrupt if you use for more than few minutes. Can make require a expensive reagent, can make it stop all mana regen and make you lose mana gradually, there are "N" ways to not have a broken temp immortality skill without cooldown.
Same for transformation skills, werewolf/bear form on D2 is amazing exactly because has no cooldown.
Cooldown = shit.
This is a reason to why most classic RPG's doesn't have cooldowns. Baldur's Gate 1/2, NWN1/2, IWD1/2, M&M VI~VIII, Ultima, Wizardry, all golden box games, Darkstone, etc. I loved kotor 1/2 but could't play swtor due cooldowns and wow like itemization and homogenization.
edit : for the necromancer, if i need design a immortal skill for D2 necromancer, i would create a "mistform" skill where the necromancer can't use most of his spells, can't use physical attack, is still vulnerable to holy/magical damage, but immune to physical/fire/electricity/cold damage, negate all manage regen and you start to lose mana at 10 points per second at skill lv = 1 and each point invested in the skill, reduce the mana loss, that way, will be much more a situational skill good mostly for minionmancers.