how about this idea? increase the slot for creature in ritual when you advance to next mind level.
for example, Mp3 got 6 creature slot for ritual as normal now. when advance to Mp4, you get another 1 slot - 7 creatures in ritual... Mp5 got 8 slots and so on....
This way, I think it feels like we have advanced in army building since we have got bbigger army, not just the old creature in the old ritual. The more MP should mean more powerful of the *summoner* so they can process more creatures army and also..in ritual....
This is not affected the balance of the game because different mind power levels can not attack each other apart from the RPC (which I think it's bad idea for RPC to attack lower mind power level anyway)...