RTF is a global modification that replaces all towns in the game.

What if we made the caster a Gnoll as well?

That's not a good idea. We'd have 2 Lizards, 2 Gnolls, and only 3 other swamp creatures... it doesn't seem very balanced.

I have an idea to make the caster some kind of human shaman/druid (like some mages found refuge among the Lizards and Gnolls).

It's a questionable idea, but we need a caster.

We could make the caster some kind of swamp spirit (it's a bit far-fetched, but... maybe it will work?).