The way I see it, there are two ways it can be done in order to keep the balance and not make this OP as heck:
- Late-game Upgrade. Something very late in the game. It could start as an upgrade that unlocks new Divinities or Ascension Upgrades, and those new upgrades would 'increase your minions' max level by +1/2/3'. The price could be around 800T-1.5Qa
. - Early game investment. An upgrade that gives the Minions 'Ranks'. They each start at Rank 1, and if they complete enough missions to pass a certain threshold (Like completing 50 missions) they rank up to Rank 2, which increases their max level by 1. Basically something that becomes something huge late game. Since it has some potential (because Inner Power makes minion levels obsolete to most of them) it could be an Astral-Locked Ascension Upgrade that costs around 250-750B
.
The name of the upgrades could be something like 'Minion Training I/II/III' for option 1, and 'I'll Make a Man Out of You' for option 2.
Feel free to say whatever. I'm open for changes