A first step would be moving existing rules into using .SRCINFO using a separate class SrcinfoRule which should in the future become PKGBUILDRule but with the new class which should be able to gradually port things.

Another rule which cannot be done with a .SRCINFO: PackageFunctionsRule reads the pkgfunction field in subpackages.

https://gitlab.archlinux.org/pacman/namcap/-/blob/5ac0244522e35d727a0b3ddb3c6b028ad82eba1e/Namcap/rules/splitpkgbuild.py#L10-26