I'm not too sure a horde mode gametype would work in For Honor. Soldiers pose little threat, and Captains are just a distraction. The maps are fairly small so objectives would not add much variety. As for actual opponents, theres really only 4 levels of bot. Level 1, Campaign, Level 2, Level 3. All they could really do to add difficultly to increasing waves is make them more spongey, which gets tedious very quickly.
This could work by starting with regular minions and captains then throwing bot AI which increase in difficult level over the waves. To mix it up they can add special events such as catapult barrages which would cause the catapult feat to be launched throughout the battlefield as you fight. Increasing health is not always ideal, but if you make it with reference to Gears of War it might work. You go through waves 1-10 then it resets increasing the damage and health of the AI.