Denethor implemented via such mechanics was surely lore-accurate and a very smart of a system, but sometimes one has to accept compromises, in order to have everything work in the most decent way. In this case, that past type of implementation caused quite disturbing bugs. Furthermore, to answer your suggestion, I don't know if that eventuality would be feasible; nevertheless, in my opinion, we might then risk to have a replica of Saruman's feature in the game, which is something that ought to be avoided. I therefore believe, due to the great references to the films, that Saruman's ability had better remain his own exclusive possibility.
Denethor's current design is very unique anyway, although it is so under a different perspective; especially, I find his characterisation as a noble lord (who may summon his personal guards) and his stance-system marvellous additions to his concept. Henceforth, we had to renounce the previous citadel-feature logic, but we have, on the other hand, another well fitting concept at disposal. In a sense, it's similar to what we've been proposing in regards of Old Bilbo and Erestor for quite a time: as a solution for issues and unpleasant implications, Erestor could be made a permanent hero and Old Bilbo could be permanently introduced in Rivendell too, each of the two retaining their proper supporting nature.