I few things I noticed about Hyborem while conquering him:
1) He
really likes windmills. This is generally bad, because they give him useless food, but I've never seen him build a mine. Maybe he could be given a unique 'Infernal Windmill' (need a better name) that gave no food but extra gold? Maybe he could be forbidden from building windmills as well? Maybe preferring to build cottages on hills/grasslands?
2) I've noticed that he doesn't like to raise his cities beyond 20 or so. Probably a sound idea with the happicap, but it does make the swarms of manes that flow to him worse than useless, as (I think) he pays upkeep for them. Why not make it so that each unhappy citizen in Hyborem's empire gives 1

? As hell's cities fill to bursting, it overflows its borders to take over neighboring nations.
3) Also, I've seen him with 50+ manes wandering around after some purges I'd been doing against AV followers. If you don't implement idea 2, it'd be nice if he had some way to make those manes into something useful without burning tons of gold for upgrades.
4) Doesn't it seem odd that you can capture cities full of undead souls and put them to work for you? I'd give Hyborem sort of a 'reversed scorched earth' trait, so that any cities you take from him would have to be razed.
5) Also... broken lands (grasslands) are pretty crummy for Hyborem. While the plains replacements give him an extra hammer each, the grasslands replacements are pretty worthless. Any way to remedy this without making Hell terrain too powerful for other AV civs?
6) I've also noticed his city population
dropping at times. I have no idea what causes this.
7) Could he be made a tad bit more aggressive? I've seen him bordered by some wimpy Luchuirp on one side and the ocean on the other, with literally hundreds of soldiers ready to fight, yet his current war with Basium means that he doesn't want to declare a second war.
Phew... next game, I'll play as Hyborem to see how it runs on his end. These are all just things I've noticed playing
against him.