Let me give you an example: At some starting dates - Alexiad I think - King of Castille starts game completely controlling Galicia and Leon, but he does not have Kingdom of Galicia itself. Since Castille is primary, Galicia drifts into it (but Leon does not). Once it does, it creates rather ugly situation, with Leon stuck between two parts of Castille.
Solution 1 - make ai autocreate additional kingdom titles.
Solution 2 - duchies drift only to neighbour Kingdoms (and Kingdoms only to neighbour empires), unless kingdom is titular, then duchy which contains it scripted capital might drift into it.
So in case of Castille-Leon-Galicia, Galicia will drift into Leon, since its neighbour unlike Castille (and owner has Crown of Leon).
This way there will still be dejure drift, but results will be less ugly like Egypt drifting into Ireland.
Also, it would nice if, for example, once Cornwall finish drifting into England, it starts counting down from 99 years drifting back into Wales - so if King of Wales quickly reconquest Cornwall, he does not have to wait 100 years, but only 1, for it to drift back to Wales
Solution 1 - make ai autocreate additional kingdom titles.
Solution 2 - duchies drift only to neighbour Kingdoms (and Kingdoms only to neighbour empires), unless kingdom is titular, then duchy which contains it scripted capital might drift into it.
So in case of Castille-Leon-Galicia, Galicia will drift into Leon, since its neighbour unlike Castille (and owner has Crown of Leon).
This way there will still be dejure drift, but results will be less ugly like Egypt drifting into Ireland.
Also, it would nice if, for example, once Cornwall finish drifting into England, it starts counting down from 99 years drifting back into Wales - so if King of Wales quickly reconquest Cornwall, he does not have to wait 100 years, but only 1, for it to drift back to Wales