This is less so a request for improved moddability, but rather a request that makes some modding much less frustrating.
1) When a title is gavelkind, primogenature, and ultimogenature and has absolute cognatic, enatic-cognatic, or enatic, make it so that AI rulers will not accept an unfavorable marriage with their first and second in line daughters. In enatic and enatic-cognatic they will actively seek out favorable marriages for the first three daughters in line to the throne, and are willing to accept matrilinieal marriages with sons regardless of their position in line to the throne, but are less willing to accept such an offer, same as women high in line for the throne right now. If the title is elective with absolute cognatic, then the eldest three children should be given preferential marriages in addition to the heir. If elective enatic-cognatic, or enatic, then the eldest three daughters.
2) Change succession by primogenature, gavelkind, and ultimogenature from always granting titles to the father in the event of there being no children on succession. It should go to the previous holding parent first, then the father if agnatic or agnatic cognatic, then the mother inf enatic and enatic-cognatic, and if absolute-cognatic, whichever parent the that the child got their dynasty from.
3) Make gavelkind's split titles share the gender law of the original title.
4) Fix gender law defaulting always to agnatic-cognatic, unless the ruler doesn't meet the potential requirements for agnatic-cognatic, in which case the title is always agnatic, even if the ruler doesn't meet the potential requirements for agnatic.
5) Restore title granting to default to the gender succession law of the granter. This forces a great many hoops in order to make sure that duke and count titles in Germany stay agnatic.