So many good memories playing Twisted Metal lol! I played hours of that game on my PS1, i think i even burnt a disc after playing it for so long.
I'm going to go with Fallout here, imagine the posibilites of battle royale on the Fallout universe, fighting on the destroyed DC, the weapons in fallout are pretty cool, I imagine a Fat Man would be a legendary weapon of sorts, even with just 1 ammo it would be insanely op! but i think the battle royale mode would shine even more if the battle happened on a vault, it would be like Fortnite or PUBG but with claustrophobia, every corner you turn around would be a possible death since there could be a player there waiting for you and the overseer office could probably have tons of good loot, so everyone would be rushing there at the beginning and then spreading out.
Don't even get me started on power armors, a total game changer if you get them too! Limited use of course, like in fallout 4 but with only one fusion core that depletes faster or it would be too op.
Instead of Thanos as an especial event, there could be a deathclaw invasion in the area you are in or a Brotherhood airship suddenly crashes on the battlefield, full of good loot, totally insane, would play it 100%. I don't know what to expect about Fallout 76 though, many rumors going around but i will wait until an official announcement happens at E3!
I was going to say Fallout but you beat to it! I was in the middle of my explanation, saw your post and scratch everything lol.
Things I would add to your idea is, being able to hack turrets, Sentry Bots, Mr. Gutsy, and Protectrons, to attack your opponents.
That would be so cool too! every in game element should be incorporated including hacking robots and turrets to your benefit!
runner-up