Perhaps recovering the knights of the unicorn? I was thinking of sub-optimal knighthood order (level 40-41 maybe, so dont get to recruit them "freely" but their stats being akin to a commoner troop). Just for the sake of it.
Also i was thinking of allowing the player to recruit small spawns of bandits (below 20-30 in numbers) when having negative honor (<(-5)). This last bit is something from good old SoD (Sword of Damocles) for original M&B that was pretty useful in the beginning of a game.
Some people have mentioned that super-mega-ultra spawn for engame, which I would love (i wished for Sindar in PoP since they were first mentioned/conceptualizaed, I'm a tree hugger I know).
I recall another feature from SoD that consisted on sending top tier nobility on some treasure hunts (funded by their monarch, you) to foreign lands. For PoP, i was thinking of something close to it, but expeditions to Mettenheim, Melitine (empire?), Barclay... in order to get some pieces of equipment or even hire some troops (kinda like the heartbeat quest). This wouldn't of course be sustainable economically, but could help to get that particular piece of gear or get a particular theme for the army (i love melitines).
And lastly, an idea for a knighthood order quest came to my mind. Some kind of "clear the honor of the order" where you have to fight a small group of rival knights or order knights (5-10) alone (only for the actual battle, no need to clear the party). Like the order initiation combat where you fight 1v3 or the ambushes where you are 1v6-7.