Obviously I'm talking about a crossover with RWBY but hey here is my take.

Madness Combat.

Now why? Easy! You can do story's of all types! 

If you want something more relaxed you can use a lot of the fanon stuff. Having Hank and co interact with Hunters and Beacon. The AAHW could be elite mooks for the characters to beat and you can have the Madness people be occasionally brutal taking by surprise the people of Remnant.

You can also go full on lore accurate Madness and have the story start with the corpses of the twins falling from the heavens onto Remnant and have the Auditor reveal himself as the primordial angel of order and that a place such as Remnant shouldn't exist.

Now because Auditor can't use his full power against mortals you have career against the clock before auditor finds a way to get Remnant to be nothing more than ash. Full on brutal fun and madness.

Of course for the people more fan of military stuff and strategy there is always the Nexus Core and N51 which you can put against Atlas (although for Nexus you would need to give them a reason to not just nuke them with dissonance)

Stuff like Ruby's silver eyes could also be explored more as say something that Auditor wants to weponise because it's technically order magic. Etc etc etc.

What do you guys think is the best franchise for a crossover fanfiction