Ive been thinking about this as this is what I think, similar to what ive done for the Archmaesters:
-Kingsguard could have a titular county/duchy with one barony in King's Landing
-Any member of the Kingsguard gets a special Kingsguard trait
-An event that fires every year or so for the commander of guard checking how many characters have the kingsguard trait (similar to the Archmaester one I made)
- If <7 a new unlanded character could be selected at random from westeros, with high martial high prestige characters being more likely
- This selection would be then presented to the King for approval and then a further event for the character giving them the choice (less likely to accept if an heir)
-If they agree they are moved to the Kingsguard barony and given the kingsguard trait (and hopefully some cool portrait graphics
One problem with this is that unmarried courtiers over the age of 40 are automatically culled by the game. if we can figure out a way to give them an honorary title by event this is prevented.