Something that would make Hyborem stronger and the AV weaker: Change the Hell terrain for grasslands. I'd suggest either 1

/1

or 1

/1

. Make it so that the AV doesn't get such massive cities, and gear Hell terrain more towards Hyborem's needs. In fact, you may also want to make obsidian plains give 1

/2

or something awesome like that. Basically, the point here is that Hyborem will spend citizens working empty grassland tiles just because of his AI. Remove the useless 2

tiles, remove the problem
Secondly, bar him from adopting Sacrifice the Weak. It is quite useless for him, just giving him a penalty to GPP, and inducing him to sacrifice precious population points on buildings...
very harmful, and it doesn't make sense for demons to be using such a thing anyway.
Thirdly... Hyborem needs to get out more. I'm not sure what you can do about this, but having him wait a few turns to get his empire started, then declaring war and going reaving once he's gained a combat promo or two would be a real nice strategy.
Finally, as an oddball request/question, can we expect Hell terrain for water and cold terrain any time soon?