Not much solid info here, but some of it was kind of interesting;
http://www.starwarsnewsnet.com/2016/01/the-rumored-cast-of-star-wars-episode-viii.htmlApart from Trevorrow confirming that both Mark Hamill and Carrie Fisher will be in ep. IX, plus the already discussed re-appearance of Harrison Ford, it seems that Max Von Sydow might return as well.
If so that confirms that full fledged flashbacks will be used as a story device in further movies. In my opinion it’s something Star Wars needs.
People have already speculated how Han may make an appearance in a flash-back, a hologram, or even as a Force ghost (the last one I personally don’t buy)
Or in a hallucination. Kylo Ren is mentally unstable.