Lord Brutus said:
By saying "THE ULTIMATE multiplayer Bannerlord experience", you're setting a pretty high goal but it seems to me, you're not beating a dead horse, you're beating a horse that hasn't been born yet. How many players to you envision participating simultaneously?
The number is unlimited. You can have 10 million players simultaneously.
Not on the same battlefield of course, since the Bannerlord servers will support however many it supports. Will it be 256 players? If so, then yeah, that many. How do we have 10 million players participating then? Well on the overland map, the number of lords that the game COULD support is... I don't know! Do you? This is one reason I speak of a truly massive map size. Say you have 200 factions (independent lords, barons, counts, dukes, kings) with an average of 10 lords each, that's 2,000 lords (players) moving their armies on the overland map simultaneously, visually in (semi) real-time... "Visually" because of course the fog of war prevents you from seeing very far around your party or parties. Remember, it doesn't have to be just one warband, it could be multiple, as in one plus small scouting parties, reinforcement parties, and so on. Or maybe the map is the size of actual Europe, the UK, Western Asia, Northern Africa and the Middle-East and that brings us to a possibility of many thousands of lords?
So, wait, say we're limited to a war with "only" 25,000 concurrent players moving their war parties on the overland map? What if we have 50,000 players wanting to join? Obviously you can have any number of "Medieval War for the Known World" events ongoing at any moment. You join a war, you join a faction, build your warband with your share of your faction's resources, out of your faction's culturally appropriate troop tree, you pay their maintenance from your war funding from your kingdom and whatever else you earn from winning fights for example.
It could be that everybody starts with only recruits and they get leveled up either through training or through battle experience. Then just like in SP their equipment, stats, skills and cost all increase. You could have a small number of elite troops, a large number of low tier troops, anything in-between, you decide. Only, don't go bankrupt. There could even be capturing troops and, upon bringing them back to a faction city, adding them to the kingdom's pool of manpower... or simply ransoming them for some cash.
So you have thousands of players moving their warbands around on the overland map, and then enemy warbands meet up, triggering a battlefield. Then for the next half-hour of real time, other players can bring in their forces. The setup I have seen didn't have a time limit, players could add troops during the whole affair, but whatever. Then after that, say 30 minutes of "getting ready to fight" time, the battle opens for being played on a server. When the server populates enough, the battle starts, say 800 Germans against 1100 Bohemians, with whatever troops were brought in by the players. Field battle, Village, Castle, City, whatever. The multiplayer match begins, with whatever number of players the Bannerlord servers allow, soldiers are killed, wounded, knocked out, captured, and so on until one side wins.
So at any given moment, as a player, you may move or not move your warbands. Then you look at the map, see what battles are open to populate for your faction, you bring your buddies to where you want to play, you fight that battle. The frontline moves. You have succeeded or failed to bring your faction closer to victory.