The ability to tailor yourself in the direction of specific damage types, or to be better at tanking, or to be faster would be an interesting dynamic, especially considering the way that damage types work; it may be advantageous, for instance, to deliberately not purchase better armour if the other team are speccing into blunt damage and to instead focus on high damage glass cannon builds.
The idea of a "progressing" battlefield, where both sides are accumulating gold and upgrading themselves in the direction of their choice, would be a very unique take on the MP compared to both Battle and Skirmish, which offer round-based gameplay. At the same time, it wouldn't take away from the anticipated "teamfight" engagements which teams co-ordinate in Battle and Skirmish, what some call a "crash" when the infantry engage one another and the support classes come into play. There would still be a need for each side to co-ordinate themselves and bring all of their players together in decisive fights, only that these fights would be more organic, less frequently involving the entirety of each team, and over objectives on the map which give more minor benefits than flags which simply win rounds.
There are definitely ways that Mount & Blade lends itself to this sort of gameplay and other ways in which it does not. "Lanes" could simply end up with players duelling without real way to disengage except by both sides choosing to back off. Altering that would require the addition of something like a "roll" or "dodge", which could be put on a large cooldown to prevent its abuse in combat, but the addition of which would be a serious step away from the tradition of M&B gameplay. And, if one were to go that far, there's the even further step of straight up abilities of the sort which exist in MOBAs. Miring a strong combat system in asymmetrical abilities could be a serious source of frustration in a lot of circumstances. A For Honor-type mode with a MOBA-like economy and XP system, centering the M&B combat system, if carefully constructed could be something genuinely interesting. Not that it is anything within the realms of possibility until MP modding is made available.