I support the 1v1 Mode suggestion.
If Duel Mode:
Let players join an ongoing 1v1 match and spectate. While only 2 players is required to play, other can join up and wait in queue. Kind of like the Last Man in T:A Arena matches. They should be able to roam free in a walled off area, able to watch the duel, but not interfere. They ofc can chat, cheer and taunt as much as they want to.
Then if possible make the spectators be in a queue, that randomly picks a spectator as the next combatant, after the previously duel has been finished. The newly picked spectator then have to fight the winner of the prev. match. Kind of like Chivalry's 1v1 duels.
Make weapons suited and fun for 1v1 and maybe remove the Healing Tool for 1v1?
I have not really any suggestions for a 1v1 Race Mode, but I think that would be fun too. Yet it would be even more fun with 5 players, when talking about a race.
EDIT: If possible make it so the spectators can rank the match and give points to the winner. So when the duel is over, every spectator can choose from 0-10 points, which is then put together to a final score. Just for fun.