I can picture dark angel and space wolf captains duelling one another to settle the feud started by the Lion and Lemans fight.

Chaos and Orks more so, be amazed if we see them on Arkhona at all.

maybe we could have an instanced area, like inside the ship were those duels could happen.

For instance, the Astartes have the training cages. Friendly duels between battle-brothers are widely mentioned in the Horus Heresy series.

No dueling on the battlefield. we don't want this to become like Wow where you cant go to cities becaus you get spamd by duel requests.

That can be easily arranged, both parties could just ready up for duels from their menu.

Oh, moreover, any individual members of Dark Angles and Space Wolves could repeat that duel, and often seek for it ^^ At least that was the impression I got from the Deathwatch RPG by Fantasy Flight Games. We should have all that cool and medieval stuff in the game no?