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.
Thoughts?