I think the games workshop would benefit from having the main campaign added to the editor. This will help players learn the editor basics and give them high quality templates to modify or take inspiration from. Starcraft 2 does this and it has a thriving community of modders who put amazing spins on the campaign missions. This would also give people an opportunity to get their NG+ mode they have been requesting without the developers actually having to add that themselves.

The only scenarios I have by default seem to be the tutorials, which are fine but not a fully fledged mission.

A few scenarios that users could make from this,
NG+
Main Game Merc Units Only
DLC Feds only
Main Game Mechs only
Main game but with all versions of bugs on every map
Main game but remixed with all units available and remixed enemies

Also, any kind of unit editor would be amazing. In the same way that the editor has kept Starcraft 2 alive so long a more robust editor for modders would do wonders for this game.