Before saying this is a dupe, please read the post.
Just Build Mega/Showdown would be a game with 16 players, without teams. The game would have three building/voting phases.
In the first phase you would get a random theme, like the regular just build. However, only one other person has the same theme. Everyone else has a different theme. At the top of the screen in the custom UI, you would see who else has your theme, basically your opponent for this round. You would have 3 minutes to build the best thing you can.
Once the 3 minutes are up, everyone will have hopefully finished their build, and it is now time to go on to the first voting phase.
A random player is selected, and has their build placed in the vote zone. Their opponent, or the player with the matching theme will also have their build in the voting zone.
All other players will have 20 seconds to pick which build they like more. You can pick from slightly better, and much better.
Slightly better will grant 1 point to the player you voted on, and much better will grant 2 points. The player with the least points is eliminated.
This process will continue to each set of players, until 8 players have been eliminated.
In Phase two, the same thing happens, except this time you would have 5 minutes. The voting phase would begin, and reduce the player count to 4 players.
In Phase 3, the remaining players would all get the same theme, and have 7 minutes to build.
In the voting part, two random players will be selected, and the other two players (and the players that are spectating) will vote on which build is better. Then, the other two builds are put against each other, and people will vote. Finally, the two winning builds will be put against eachother to declare a winner.