Ok so as others have said, it would start with him getting the stones in space. Now there could be some exposition of him and Death or his motives, or it could just kick in to gear. He would start at Odin's Vault, bringing in Thor and maybe Hulk if he's still here. He easily defeats them. He raids the Nova Corp on Xandar, the only ones to fight him are the Nova officers. No match. Then he goes over to the Collector. This brings the Guardians in, seeing as by now they have heard of the threat he poses and his plan. They put up a fight, it's the most match he's had but he ultimately defeats them.
By now, Thor would have made his way back to Earth. He warns Strange, seeing as he is probably the most powerful impartial Avenger there is. Strange calls everyone to order is his sanctum (man that would be so cool to see), and him and Thor describe the threat ahead. There may be some verbal arguing between Team Cap and Iron Man, some may even choose not to fight, their prejudices blind them. So they're down on numbers. Their two powerhouses, Vision and Strange, are in possession of the two stones. Thanos arrives. Strange puts the Avengers and Thanos in the mirror dimension, nobody else can get hurt. A major fight ensues, but even they together are not powerful enough for him. He rips the mind stone out, he tears the Eye apart, and he has five. Nobody's dead, but they are hurt.
The Guardians arrive, too late. Could be a humorous moment ( Quill: "Don't let Thanos get the stones!" Stark "Yeah no shit- is that a tree?") Introductions are made and it is the Guardians who bring all the heroes of Earth together, their objective position shows everyone just how stupid they're being, fighting against each other, when the real threat is and always has been out there. Captain America is worried they might be too late. Strange says otherwise, the final stone is still out there, the soul stone. They need to get there before he does. And it's going to take all of them to fight. This is where Coulson could come back in, SHIELD might have just the ship they need. It'll basically be a spaceship version of the helicarrier, can travel at hyperspeed. Cap tells Coulson he feels bad, leaving Earth defenseless. Coulson responds "Don't worry, we've got some heroes for hire." They leave for space, with every Avenger and Guardian.
The rest could be a race for the soul stone. Cutting between the Avengers and Thanos. Here we can get all the interaction moments we've always wanted. Once they arrive, moments within each other, so does Adam Warlock, protector of this stone. Thanos waves his hand, and Warlock is gone. He threatens the Avengers not to cross him, they don't want to see what happens. They don't stand down, so he snaps his fingers, and Spider-Man and Groot drop dead. He hit the hearts of both teams. He can end them all, so Cap tells them to back down. He rather nobody else die. Thanos gets the final stone, and he has them. Now of course they defeat, but I don't want to write it all now, frankly can't think of how they do. I know i wanted Warlock to stay hiding in the Soulworld that the Soul Gem has, and maybe others join him? I'd also like Thanos to make the heroes fight each other to the death, since he senses their inner divide, and we get the real civil war. I think it would really be Strange and Warlock who ultimately bring him down, but I've already written enough.