I think, if they do Ragnarok right, there will be some more Thor movies beginning 2020.
As i wrote in a different thread, i think, it is too early to bring Ragnarok - because they should show more stuff about Thor and his world before they go into this!
And, in movies, what should come after Ragnarok?! Can it really be better - or does the franchise go downwards, as they already showed the "peak"... i think its not a good idea for the franchise overall, to bring this theme so early!
But, if they do it right, there will be a fourth film for sure. And then it will depend, if the fourth movie is a success, and what they still have to offer with a fifth movie...
As they plan already till 2028 - i do not really think, that they will do this without Thor, one of there most successful heroes
And if all works fine, we see around 2028 the sixth solo-movie from the Asgards...