I thought up another Last Stand idea (because, honestly, this is a lot more fun than other work, lol) that, while maybe more "Fair" than "Circus", could still fit into this theme.
The battle arena takes place on the top part of a Ferris Wheel. Players can stand on the roofs of the cars and climb on the struts for the wheel itself, but falling to the bottom of the screen (or even riding the roof of a car too far down) is a kill zone. Each of the cars swivels from the center of the roof (just like in reality) so in theory they remain upright as the wheel turns, but the activity on top of each car may also cause it to roll back and forth on its pivot.
The wheel itself will alternate between being stationary for a short time, then turning in one direction or another (the direction and speed could be randomized and adjusted based on how far along the round is, and there could even be some rounds where it is constantly in motion). This forces players to try to fight each other while hopping from car to car to stay in the battle area. There could also be large balloons floating around that you can stand on (and/or grab ahold of) as a way to temporarily get off of the wheel (or even recover from riding on top of a car too long and getting too close to the bottom of the map), but if the balloon has too much weight on it the balloon starts to sink (giving an opportunity for other players to sabotage them as platforms by deliberately sinking them).
So what's the "wave" hazard that makes the stage even more lethal and forces an end to a round one way or another? Fireworks. Maybe the stage takes place at night, or it alternates between a day and night cycle like Winter is Coming, but while it is night time there can be increasing waves of fireworks displays that explode in the area around the battle arena (maybe they even destroy balloons that they hit, or the balloons go away during a display if it's an alternating day/night cycle). Aside from being pretty they could behave like the blast radius from bombs (possibly with the size and strength varying depending on what kind of firework it is), stumbling and moving people around or even knocking them out if they are close enough to the blast. As an added bonus maybe the spectators' bomb throwing option is replaced by launching a firework (or small group of fireworks). The final wave is a grand finale that will set off a huge display with the largest blast radius fireworks, making it a struggle to find a place to survive and not get blown off the map.
That idea might be pretty taxing on the processor and GPU, but the sheer amount of physics craziness potential seems like it would make for an incredibly fun and thematic map. 🙂