I have a different solution. Let me preface this by saying that this solution I am proposing is due to the historically extraordinarily low activity of non-standard mode BNs as a group relative to standard (since there's just many less maps and also the activity standards seem quite a bit more lax to me).
I think that one nomination should be required for each mode, and then one nomination for the mode with the most diffs. So a ENHIX - M O IO spread would need a taiko icon and two standard icons. I say this because finding two nominations for a game mode in which you as a standard mapper do not play and are not part of the community for is really hard - much harder than finding two nominations for your own game mode. That coupled with the activity level of the other game modes would make it very hard for hybrid sets to get ranked in a timely fashion as the host would struggle to find 4, 6 or 8 BNs, the majority of which would be people they don't talk to usually anyway. This can be implemented along side the separate mode nomination counter - each mode needs one nomination then a "capstone" nomination to qualify it.
This solution provides an optimal balance between quality control concerns and difficulty of ranking the set in my opinion. Maps like CBCC or Black Rover or whatever still need the two standard BNs and it doesn't become prohibitively painful to find all the other mode BNs either.
Two per mode is better than making them unrankable, that's just annoying both for people who prefer to GD than to make sets, and for people who map two game modes.
I think that one nomination should be required for each mode, and then one nomination for the mode with the most diffs. So a ENHIX - M O IO spread would need a taiko icon and two standard icons. I say this because finding two nominations for a game mode in which you as a standard mapper do not play and are not part of the community for is really hard - much harder than finding two nominations for your own game mode. That coupled with the activity level of the other game modes would make it very hard for hybrid sets to get ranked in a timely fashion as the host would struggle to find 4, 6 or 8 BNs, the majority of which would be people they don't talk to usually anyway. This can be implemented along side the separate mode nomination counter - each mode needs one nomination then a "capstone" nomination to qualify it.
This solution provides an optimal balance between quality control concerns and difficulty of ranking the set in my opinion. Maps like CBCC or Black Rover or whatever still need the two standard BNs and it doesn't become prohibitively painful to find all the other mode BNs either.
Two per mode is better than making them unrankable, that's just annoying both for people who prefer to GD than to make sets, and for people who map two game modes.