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So, basically, you suggest here, and I'll forward it to him via email!
One and a half Banshees for Necromancy – easy. :D
Secondly (strangely enough), my suggestions align with previous posts, namely, strengthening specialists based on creatures.
But not by simply increasing the growth rate.
By the way, I didn't pay attention to this issue. In the Dushman mod, he already created a specialist for unique units, namely Mirabelle with her pirates. Yes, the idea there isn't about conversion; they just appear. But it's probably easier to balance that way. Specialists for Jellu and Dragon for collecting creatures from different cities and converting them would be more specific in Multiplayer, as their usefulness sharply changes depending on the presence of certain cities on the map, ranging from simply overpowered to mega-overpowered.
And all this is not free.