Or at the expense of the king if enough barons get together; it all depends on who does what and who is friends with who, but having oaths of service and oaths of this and that would still, in my opinion, be a nice roleplaying touch.
The problem is that if it is RP only, well then we can do that right now. Nothing stops people from recording oaths on the wiki and causing a ruckus if they are broken. If they are game mechanic supported, besides adding yet another system when we are in the middle of fixing/balancing existing systems, it would be a massive job to make them flexible enough to satisfy people.
My advice on this has been the same for a long time, if a realm wants Oaths, they already have all the power needed to implement them. It does however require work from those holding positions of power, notably the Judge who probably ends up enforcing oaths and perhaps recording them, and the Dukes and other council members making enough noise to convince people to actually codify their oaths and write them down.