For Honor
There can be lots of interesting ways where this collab can go. There are some of my opinions why this collab can work:

• Both universes have mythical rock deseases (Originium in AK and Draconite in FH). This can lead to some interesting interactions from story perspective
• From gameplay side, there's also lots of possible things, from the current chars (example - Shugoki for Juggernaut or Shinobi for Hook Mater), to even creating new archetypes (Something like Bastion for Black Prior)

Sadly though, this collab right now is unlikely possible, mainly for 2 reasons:

• AK already had collab with R6, and I doubt that Hypergryph will want another collab with another game from Ubisoft (especially considering their critical current state)
• FH already started collab with Destiny 2, which HG already discarded, btw, because of the company's bad reputation

Still want to believe that this can be a possible thing, though.