i would like to suggest three choices:
1. reduce the diff limit to 5 (without taiko/ctb) or 7 (within taiko/ctb), and the creator must make at least one insane diff if there're no less than two insanes, or one hard diff if there's only one insane.
that because i think creator must contribute the largest part of works in a mapset, or it is meaningless. Allowing GD for guest mappers who really love the song but have no enough time/ or similar reason that cannot be make a full mapset.
2. Creator must make the diffs which have a proper diff spread, and allows unlimited GD amount.
if a map has a proper diff spread within, it would make all players enjoy it. So, additional GDs could be considered as "extras" for pro players. eg. creator makes E-N-H-I, and then add 3 insane level GDs for pro players, it is acceptable.
3. Remove this rule, and ask MAT/BAT to handle it
all will be depended on each single cases, staffs will decide it and won't be disturbed by a general rule.