I wouldn't directly call it a "hardcore mode". The idea of hard core drives away causal players that can be successful too. I think the best way to think of it as a mode that mirrors realistic middle age warfare.
I'd say have a sever with rules seperate from the non-conquest server that allows for Home regions and castles to be lost. If you lose your home region you keep part of the resources you had and are given the option to resettle on another continent.
To keep it fair, allow players to designate a 4-6 hour window when they can be challenged for territory control isntead of just raids for resources. If that isn't acceptable, then allow raids at any time but only allow conquest challenges when both players are online.
To prevent instances of mass Realm on Realm warfare, only allow reinforcements from players within a certain distance of the battlefield or within a certain number of provinces away.
This makes it fairer for the casuals who can't be online all the time.