Endgame scenario suggestions
For replayability / interest, I think it would benefit the game to tune endgame scenarios a bit. Most of them require you to migrate to the central old world (Vampires, Dwarves, Greenskins, Tomb Kings, etc) making the campaigns repetitive. I think the overall idea of them is great but some tuning is needed to keep variety in campaigns.
\-Victory conditions where you have to capture critical settlements (Dwarves, Chaos Dwarves) are better than "eliminate all factions" conditions. The player is more in control of the victory here and doesn't have to go hunting in annoying places.
\-Make the rewards progressive (like Books of Nagash). As you capture each of the 6/10 required victory settlements, add more global bonuses on a scale similar to Books of Nagash. Gives you constant goals in the lategame where you've fulfilled all of your faction's goals already, and a rewards drip as you work on the endgame crisis.
\-Add a capstone battle to at least some of them. It would be best if this scaled with the overall endgame difficulty slider and would serve as a dramatic final challenge to you faction leader's doomstack.
\-Add some mixed-race enemies like rogue armies. These could be vassals of the big-bad or they could themselves be the big-bad. I find these create some really interesting battles with the hybrid rosters like Scourge of Aquitaine or Vashnaar's Conquest. Imagine armies of Skaven+Dinos, Greenskins+Vampirates, DarkElf+Slaanesh, etc.
\-Give your faction leader a curse which makes invading armies spawn near them (like the Wood Elves invasions) with a 5-turn warning. Defeating these should automatically give a fun reward like a blue+ item or a dilemma award. Makes you feel constantly engaged with the crisis, without spawning random stacks at annoying minor settlements. Sometimes these could be the rogue army types mentioned above.
\-Make the endgame factions attack in a scripted, coordinated manner (like the Chaos Invasion used to be). Every 20 turns, they spawn a wave of 5-10 armies that attack a coordinated settlement. Perhaps the player could have some type of spy mechanic where if they engage with this mechanic they will get a warning about which settlement the invasion will target or even vision of the armies.