20. Xandar Falls
By the time Thanos turns up for the first time in the trailers, he's got the Power Stone in his possession, which spells disaster for Xandar, where the Stone was sent for protection at the end of Guardians Of The Galaxy.
And given that we've been told to expect Thanos to prove how strong a threat he is in the first five minutes of Infinity War, it makes sense that the very first thing we see is the fate of what remains of the Nova Corps.
This sequence will probably see Thanos invade (on his own because last we saw him he was vowing to "do it himself"), brush aside the planet's already decimated defences (the reason why he teamed up with Ronan, even though he was a volatile wildcard), take the Stone and destroy the planet as a show of power. The planet has served its purpose and while we MIGHT get a Nova film eventually, introducing him as the last surviving Corpsman would actually be a better story.