It seems like we're just replacing an potentially partisan Chancellor with an arbitrary and capricious rule. What basis in RP or reality is for the 'age' of the settlement to determine its recruitment order? You could easily have a war in which one side owned 5 settlements that were 'old' and the other side owned 5 settlements that were 'new.' This rule could end up giving 5 units to the first side and none to the second, even if the RP circumstances dictated that the people living in the 'new' settlements would be more likely to support their Lords.
If we're going to have the system prejudice one side over another, at least make that decision subject to the political game. Both sides have a chance at wooing the Chancellor over to their side or putting their own man in office, but your proposal will simply prejudice one side in favor of another without any method for the prejudiced side to improve their situation, no matter how well they politic and prepare in advance.
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