Explicitly advertising multiplayer might help draw in players who normally only play singleplayer. The EA release trailer contains some MP siege segments but it is not actually identified as MP anywhere, so I think many people just assume it's SP. They probably shouldn't spend much resources showing off skirmish, clans or matchmaking features as those would probably actively turn away new players. I think what would entices new players is epic 200 player sieges, organized shieldwalls and cavalry charges.
Once you get people to try the game, you also need to get them to stay. We know that a progression system is in the works, but we don't really know how fleshed-out it will be. IMO, it would need on the same level as For Honor's progression system in order to be worthwhile and manipulate players to keep coming back regardless of how much fun they're having.
I think looking at the gamemodes we currently have and that we know are likely to come in the future, only siege directly caters to casual players. I think an invasion gamemode (players vs lots of AI) would be another gamemode that could introduce casual players to MP and get them to keep launching it. On the topic of BRs, it seems like any BR that's locked behind money is likely to fail in a few months. Bannerlord has a large amount of players, so it might be viable, but when you consider the scope of this game it seems likely that it would just end up as a dead gamemode shortly after release. I guess an alternative would be to release a standalone BR mode for free, and hope that that convinces some people that would not have otherwise bought the game at all to try the regular MP.
Another interesting idea is to introduce a metagame to MP. Something similar to For Honor's faction war, where players can choose to join one of the factions, earn resources from playing matches which they use to attack and defend territories on a map of Calradia, maybe have events based on movements of the map and ultimately reward players with cosmetics, badges, etc. when their faction wins.