Technically, we're able to roleplay using only ascii and a terminal screen. In order to be widespread, roleplaying has to be appealing, and that can only happen if there are basic systems in place to support complex player-made roleplay.
What I think could be great starting points with high payoff would be in-game support for kingdom laws, family/bloodline system, and item uniqueness.
Kingdom laws refer to kingdom complexity: a king can dub anyone a knight or a duke, but that's rather meaningless without basic mechanics to bind them to the crown and give them special actions, or even an official feudal caste and vassalage system that provides pros and cons to each social class (instead of just playing pretend). Unique realm-wide edicts based on kingdom cultures fit here too.
A bloodline system means the ability to create and advance a dynasty through marriage, kinship and inheritance. People have been marrying in-game for some years now. This just turns that into an official game feature and gives you a name/house/family to add to your roleplay options.
Item uniqueness means the ability to name and add manual or automatic descriptions to some crafted items, and also random chances of items receiving rare bonuses upon crafting.
Buying a sword is lame and generic. The world feels more meaningful when we buy a Sword of the Seas, crafted at the Northern Kingdom, by the smith Eric of house Sinkschipp, even when quality-wise they're exactly the same item.