i like all these ideas ... but it could get a lil overwhelming. I think to keep it simple but on the same line, would be just to increase how many, of each grunt, that will come out of the portal, Along with some specific grunt buffs.
Adding different types of minions would get a lil too confusing i think.
Minotaur I - adds 1 more to each wave.
Minotaur II - increase max health by 75.
Minotaur III - Adds 2 more to each wave.
Minotaur IV - they gain +10 dmg and +5% attack speed.
Minotaur V - Adds 3 more to each wave.
Archers I - increases their health by 125.
Archers II - adds 2 more to each wave.
Archers III - Increased range by +2 (enough to be able to avoid tower dmg) and dmg by +6.
Archers IV - Now aim for vital organs. 2% chance of dealing 2.5x dmg critical hit.
Archers V - Adds 2 more to each wave.
Priests I - adds 1 more to each wave.
Priests II - adds 1 more to each wave.
Priests III - increase heal rate (how often they can heal) by 15%.
Priests IV - increase max health by 100. Priests cannot be knocked into the air.
Angels I - Their attacks now cripple, reducing movement speed by 3%.
Angels II - When they die, alllied units within 20 yards are healed for 300 health.
Angels III - +10% attack speed.
Catapaults I - increase dmg by 8 and now do additional dmg in a small radius.
Catapaults II - add 1 more to each portal.
Giants I - Movement speed and attack speed is increased by 10%.