Dr Strange this look into the future for possible outcomes and only saw one, so i'm guessing him giving up the stone, he knows the outcome.
Antman should be in the sequel, although it won't be a sequel as such.
In the comics Thanos hopes to destroy half the universe in order to impress death (the personification) although there is no mention of Adam Warlock in the movie which plays a big part in the comics
I'm guessing Gamora will re-appear in the sequel because she knew something about Thanos that he didn't know she knew (although i'm guessing this is to do with him knowing he isn't worthy of ultimate power, hence his emotional sides being revealed)
The scene with Peter & Tony at the end was quite emotional.
Thor's first appearance at Wakanda >>>>>>>>>>>
They say the sequel isn't going to be a direct sequel and it is suppose to run alongside infinity wars, maybe it will be an alternative dimension in which the Avengers will win foreseen by Dr. Strange.