I don't plan on "finishing" my playthroughs.
The most fun part of M&B are large scale battles. I enjoy them even more when I feel like I'm just a part of them, instead of a supreme commander. So the main goal of my playthrough is to build myself up enough so I can mantain (and replenish) a sizeable army. I acquire a town, workshops, companions, then I pick battles left and right (preferably as a captain), while developing my fiefs/companions until I get bored of the current version.
My dream playthrough would be a long one, spanning over several generations. But there's not enough fun stuff to do for me yet to last even to a second generation. I need more features that spice up gameplay during peace. Events, more managing of your fiefs and subjects, more interactions with other characters (especially family), perhaps feasts... More ways of RPing a medieval noble. I would love if this game was closer to a CK3 in 3rd person (with much more focus on combat of course).