I said this from about 2012, the smartest fucking thing they could have done to deal with Thanos was set up a classic Marvel villain. And the moment that shit FF reboot bombed, my thought immediately was "Cast Oscar Isaac as Victor Von Doom" because that fucker proved both in Robin Hood and X-men Apocalypse that he's basically Ricardo Montalbon reborn. The son of a bitch can ham the shit out of a fucking scene, scenery chew like a tumblrite at an all you can eat buffet and still somehow come off as legitimate and terrifying.
He's a fucking golden age actor basically or akin to Jack Nicholson without the cocaine and LSD any way. Someone who can be serious and have all the gravitas we associate with Oscar winners while still knowing when to have a fucking blast.
Any way, what they should've done was start end game not five years in the future but five or six months. With Stark, Banner, Rocket and Hank Pym failing to crack the Time Machine problem. Black Widow suggests that there's one guy they can reach out too and them all basically saying variants of hell no. Banner adding that there's no reason to suggest Thanos didn't take him out before assembling the stones, since he wasn't even present at Wakanda and no one's seen him in two years. Everyone insists it's too dangerous and after another failed attempt they opt to join Thor and Captain Marvel who are organizing a final brawl for the cosmos, even enlisting Mordu and Wong.
Black Widow heads to Latveria with Ant man, they find the place is totally fine, the Snap didn't effect it at all. They head to a castle that's rising out of a huge plateau at the center of the capital city. The doors are opened for them by mindslaved Ultron like bots and we see a myriad of magical relics and broken superhero armor from different eras. Imply Doom's found a way to retain his youth over the decades and he's the last of the golden age villains left. Cut away before they make it to the throne itself, so we only see ominous green lighting and what looks like a man of armor seated atop a throne.
Later? As Thanos is about to smash through all the heroes, their assembled cosmic allies? A green blast of energy knocks him through a Moon.
"Limited swine! Did you think your lack of imagination and your provincial ecoterrorism gave you the right to assault the very stars as if you had any purchase over them!?"
YOU DARE PRESUME?! FOOL! THIS IS THE UNIVERSE OF DOOM!