Just wanted to add a suggestion that might be a fun "mini-game" for people that wants to min/max everything.
New spells would be fun, add some balanced new spells that only drops of T1 Bosses so they stay relatively rare.
Critical hit I-VI, effect: 2-12% Critical rate (not sure if lower versions should exist or not)
Pet and Minion Mastery I-VI, effect: +4-28% pet and minion armor/Health/Damage/speed (+DR?)
Dodge Mastery I-VI, effect: +2-12% dodge (Same percentage values as blocking)
Will of the Fahakins, lasts for 30 seconds, Effect: Summons 1 out of 5 different pet/minion monster that will aid you for 15-30 seconds (Can have some really cool ability effects on the random monsters/pets you have a chance to summon and would be an ideal spell for a characters pet to use unless the monster you summon from the spell would be used for DPS increase)
Gamblers Ruin I-VI, effect: Gives a player either a positive OR negative Buff/debuff when the player uses the spell, obviously various powerfull buff/debuffs can be inserted to really incentivise players to use it, and also have quite devastating debuffs
Elemental Prowess, permanent passive buff, Effect: +10% Elemental Damage
Physical Mastery, permanent passive buff, Effect: +10% Physical damage, +5% critical damage
Aura of the Disciple, +50% all armor, +15% Maximum health, +10% All damage +3% Critical Damage, lasts for 15 seconds, 30-45 sec cooldown (Would be the #1 pet spell, not OP due to pets always using them randomly/when the spell is ready to be cast)
Ranger's Defiance, Effect: -50% Missile reflection DEBUFF on enemies (Small AoE area spell) lasts for 15 seconds. 30-45 sec cooldown (Another good pet spell or even player spell for more controlled dps bursts, not OP due to the Missile reflect curve taking into consideration the diminishing returns, so it would probably end up debuffing a totall of 20% missile reflection on a boss for 15 seconds)
Royal Heritage, +50% Gold Find, +25% Exp, +15% rare item find, permanent passive buff.
Revenge of the Nether, Effect: Drain 2000 Life to gain 200 mana over 1-3 second, 30 second Cooldown.
Can think of several more, buit it would be awesome fun if new spells were introduced that would be lootable from the higher difficulty and bosses
P.S: Critical Hit is in the TL2 Armory, and was just never released, they had planed to do it, but never did release that spell (Passive +crit hit chance)