Having read both this thread and the one on SocialHub, and some of the linked pages, I’m getting a clearer idea of what Solid is. I can imagine many of the existing ActivityPub apps being re-engineered to be compatible with Solid and vice-versa. In fact, I would go so far as to say that Solid might offer solutions to some of the unsolved problems in the activityverse, especially in areas like making user identity independent of instance, and facilitating user migration from one instance to another.
Since both standards are still fairly fresh and evolving, this will require a lot of trial-and-error, and good faith communication and collaboration between both communities. One project that would facilitate that, is federation between Discourse instances, so that forum threads can be shared between this forum and SocialHub, or between either of those and talk.feneas.org (or all three!). This is a work-in-progress already, but it could become a flagship demonstration of how to use both Solid and ActivityPub in one project.