I don't think they should add this as gamemode. In my opinion it wouldn't be a tournament if it were seen as a game mode, because you can always join a game mode whenever you want, but a tournament must be hosted by an organizer. And what is important about a tournament is that it is not organized too often and there are always a number of people.
For example, the old community events, a fun event hosted by Younisco few times a month. When they host it you can always have fun talking to people in the CubeCraft discord voice call and play game modes such as Skywars, Eggwars & FFA etc. And as for the rewards such as cosmetics and money (points I assume), it would just be too easy to get them by winning a tournament- rare stuff out of a cubelet wouldn’t really be that hard to get anymore. If you want to participate in tournaments you can always do that by joining external communities.