Over the weekend I did a screeching turn away from the companion quest work and instead started on a new lategame feature: emergent threats. There will probably only be one of these in the imagined 1.0, which will never trigger on a first playthrough but then will turn up on 30-50% of subsequent runs.
The one I'm adding will be an "enemy king" event, essentially, when you've captured 50% of the cities, an enemy noble will rally the remaining settlements to themselves as king, to provide you with a new higher level endgame challenge. The king will have some bonus features, most obviously some much tougher honour guard units and a system whereby if they lose their city but their emergent faction still has at least one city, they just get to go there and replace the new ruler. They might also have elections against them switched off and their faction or city might get protection against renown based loss (where the player can flip a city through sheer renown)? This feels mean to players who really want to complete the game in a less conquesty way though. Possibly you can only flip/election the king's city once their faction is down to a single city? That might make sense.
Anyway hopefully this will make for a more interesting endgame on some runs. I don't want to spend too much time on late-game content because I don't think many people will ever see it, which is why I'm not adding other options. Other endgame threat plans on the list include an emergent wibulnib threat where the game spams them at you in actually dangerous scale hordes, something where you either face or spearhead some kind of wrath of the forests/fairy revival as the Duke in Green chooses a champion, a dragon threat where a dragon moves around causing major destruction to cities, and an external invasion where a new faction comes in with a whole new and more powerful military unit roster. I think those are all quite low down the list though, and along with the late-game story quests they might end up getting bundled into some kind of expansion plan if the game ever has a player count to merit it.
For now, the king threat has been implemented in its basics, though the king's bonus units aren't in yet and nor is the rule for their moving out of a captured city to a new capital. So those are my next steps - then I'll try to get back to the companion quests, and then after that probably achievements and early-game quest tweaks are next on the list.