The way I understand emergent quests, they're unexpected new goals that come up because of a situation that you have in your game, but might not have happened at all in another. That's what makes them fun!
If this (hard-coded) quest is going to be like an emergent quest, it has to NOT always happen. So the player doesn't feel compelled to go for Hyborem even if it doesn't fit his game (Hyborem is an ally, he isn't much of a threat, etc...).
The Gela itself is the normal reward for killing Hyborem. The player shouldn't
expect anymore, so he can be happily surprised when he does get something more.
I'd actually advocate doing more to make each game feel different:
- Making Acheron a 50% event, Orthus a 75% event and raising the big apes' appearance to 25% of games, and maybe
sometimes have quests related to those (other than the obvious emergent quests of capturing the beasts)
- Making many more quests, each with a low probability and special conditions to happen
- Never allow a quest to happen if you already have one, but allow you to give up on a quest whenever you want, so you don't get too busy
- Maybe add a game option to make all quests certain when their conditions are fulfilled, so people can try out a specific quest; maybe a game option to allow several quests at a time for a questing game?
Quest ideas:
- If you convert Acheron's city with Rantine and you are still at peace with the Barbarians: 50% of getting the quest to accumulate X,000 gold to convert Acheron (you're adding to his Stash

).
- If a barbarian beast is in the game and you have an alchemist's lab: 5% of your researchers asking you to go and kill it for ingredients to get a big bonus in that lab.
- If you're Falamar, Keelyn is an AI in the game with a good economy and a big ape is in the game: X% of Keelyn asking you to capture the ape for a huge relations bonus and 1,000 gold (which she'll begin accumulating once you begin the quest)
- You have the tower of Necromancy and body mana: possibility of a quest where killing living units now gives your units a "corpse" item promotion which has 5-10% chance to wear off (decay) every turn and can be passed around, and gives them a "deposit corpse in conservation room" ability in the city with the tower of Necromancy. Need 10-20 of those to get a massive Flesh golem (+5

than normal?). Needs a timeframe, so it isn't too easy. Maybe 20 turns starting when you deposit the first corpse?