Doctor Strange Knows About Avengers: Infinity War In Thor: Ragnarok
Doctor Strange made his Thor: Ragnarok trailer debut when a new international promotion dropped [...]
The Time Stone
It is entirely possible Doctor Strange used his Time Stone to travel forward in time, only to return to the present timeline after discovering Thanos and his plans for Earth. Perhaps, this Strange has already lived through the encounter with Thanos and his Black Order and has elected to go to the past in an effort to stop it all.
In any case, Strange appears to have some sort of grasp on Thor's future which would inevitably be tied to the Time Stone.
There is no reason to believe Strange would travel to a futuristic Asgard and discover Hela's destructive arrival on Thor's homeland, only to travel back in time and to Earth where he will warn the God of Thunder. As far as the information provided so far by the Marvel Cinematic Universe is concerned, Doctor Strange has not traveled to Asgard, nor does he have any plans to.
The Footage
The first footage for Avengers: Infinity War (shown at San Diego Comic Con and Disney's D23 Expo), saw Thor floating through space unconsciously and landing on the windshield of the Guardians of the Galaxy's Milano ship.
Somehow, Thor's path is one which leads to him being tossed into space, defenselessly. Thor: Ragnarok director Taika Waititi has promised it is not his film which causes such an event, indicating the moment's cause will be revealed in Avengers: Infinity War and the God of Thunder will survive long enough to see it happen.
These "plans" which "destiny" has for Thor according to Strange in the new trailer likely serve a greater purpose than protecting Asgard. Perhaps Thor's destiny must deliver a warning message to Earth and the Guardians are his ride to Midgard?
More Thor: Ragnarok
In Marvel Studios' Thor: Ragnarok, Thor is imprisoned on the other side of the universe without his mighty hammer and finds himself in a race against time to get back to Asgard to stop Ragnarok – the destruction of his homeworld and the end of Asgardian civilization – at the hands of an all-powerful new threat, the ruthless Hela. But first, he must survive a deadly gladiatorial contest that pits him against his former ally and fellow Avenger – the Incredible Hulk!