What will be on page & off page SEO trends in 2020?
I think speed will play a bigger role than it ever has before, and the AMP project is either going to be ramped up or eventually scrapped just like Google+. I also think Google is putting more effort into schema and understanding and graphing entities, with the intention of building up their knowledge graph.
I'm more of Tom Peter's fan. https://tompeters.com/writing/manifestos/ has a new entry for 2020.
Edit: Adding what Tom Peter is saying about speed and Excellence:
The bottom line re 'Speed Trap' is that I think in this age of 'speed, speed, more speed,' it is in fact the case that the most important things associated with enterprise effectiveness and, yes, excellence take time."
Speed of websites, schemas, and a HUGE focus on delivering quality content for users (and not Google)
Google has actually come out and said that incorporating schemas does not affect your rank in the organic serps, outside of being in the answer box and having expanded results. Not sure I believe that one.