Great post, I have no experience or time to make mods but I have lots of ideas.
First, it would be nice to have a true polygamy mod, one that would allow me to Genghis Khan my way to victory by eventually making all the lords in the game spawned from my progeny.
A more involved mod that covers much more ground would be the addition of something of an 'end-game' invasion of Calradia by a new, outside force, much like the end-game crisis's from Stellaris. Have it set to trigger possible once one of several conditions have been met, such as so many in game years have passed, maybe a post main quest after the Dragon banner 'questline' has been completed, or the destruction of several main kingdoms has happened.
I am imagining a new faction invading from the lands between Aeseri and Khuzait, with a new city or several revealed beyond those lands, with large armies consisting of higher tier units then the rest of the factions. Perhaps a faction based on India/Siam, complete with, drumroll, mounted war elephants. Not the LOTR out of proportion size, but appropriately larger and fearsome, and able to mount a couple of archers on its ranged mounted tree line. I think this would add to the playability and a new level of difficulty and complexity, along with a new area to conquer.
Lastly, a mod adding slaver caravans, ala manhunters from Warband. Acting much like regular merchant caravans, but attacking small bandit and noble parties possibly, as targets of opportunity, while following a trade route. Add in player interactions along the lines of being able to either
trade with them, buying and selling prisoners, possibly recruiting some of their party "Mounted sledgehammers of doom, anyone?', or attacking them and freeing their prisoners into your party, for a relationship loss and giving them lease to attack your parties. In any case, they could make for a new source of quests, possibly adding in a few from towns to seek them out and have a certain prisoner released, much like the ransom daughter quest. Or even as a source to obtain the elusive sword sisters, *wink wink*.
My apologies if I didn't follow the exact post guidelines, got a bit carried away with describing what I think would be interesting. Happy Modding.