Well, my fav thing to play as in oblivion and most RPG's is a mage, however in oblivion by the time you get to lvl 10 and up, magic just doesnt cut it anymore, you have to switch to a bow or some melee weapon. You run out of mana too quick and the spells just arnt strong enough by then.
Reworking the mana system would fix it (in addition to adding more spells )