...It was always viable to play as a pure mage.

I absolutely loved how titan's quest did the magic system. It is a hack n slash diabo-esque game.

Some passive abilities would lower your total mana pool while they were active. You would gain the benefits but not have as much mana to work with. At the same time, other spells would lower you mana regeneration while they were active. For example, you could have a detect life spell that is permanently on but you would lose 100 mana total or have half mana regen speed.

Mana regen in oblivion was pretty fast once you had maxed willpower + bonuses from other sources. I rarely used magika potions.

I just find timed spells to be extremely monotonous. I was a conjurer and it was very annoying to have a summon out for only 30 sec. Of course I would make the spell longer but that would increase mana cost.
I'd like more toggleable spells that act like passives and if necessary have consequences of leaving them on.

A warrior could have multiple passives on and only have 50 mana for simple spells because he doesn't need a bunch of mana.

True.

In Oblivion you just stacked weakness to magic spells on your target and won.
Melee was easier though, but saying (pure) mages were hard is a fallacy.