Here's the cutscene I was talking about
:
https://www.youtube.com/watch?v=L1Dcj09MIuY
Where they talk about the Maximoff kids showing Thor something and he's gone to look for answers. She showed everyone something. Given it's Scarlet Witch, it's probably not a flashback. But it's the link to Ragnarok. I'm guessing Loki will appear in After credits too though.
And yeah, given Thor: Ragnarok being such a significant movie, not only Ragnarok itself, but the Infinity Gauntlet and Tesseract are still on Asgard so that just about confirms Thanos appearance. Probably linked to starting Ragnarok, too.