Thanks @josephguillaume. Yes, indeed it makes sense and is why I phrased as “focus on finding a Killer App or on making Solid a Killer Technology? Or both?”.
I think the question is more one of what can be done in addition to that to help ensure that the Solid Project flourishes? Like promotion and advocacy, organizing community events, writing and maintaining documentation, porting things to a different programming language, translate docs/articles to other languages than English, forge relationships to other communities, find funding for FOSS projects, etc.
And if doing experimentation you might consider if experiments on a lower-level of the stack might bear more fruits e.g. a devtool, a testsuite, widget library, than creating something higher up, like e.g. a chat app.
This is a very good point, and I fully agree. There have been a number of talks and public annoucements / articles that made it seem like Solid was on the verge of being rolled out across the entire web.
Communicating this well, and offering an onboarding experience for new members is one of the things where this community can offer their help 
Update: Happy to see 4 people already who want to roll up their sleeves and go for it. Nice! 
Pity to note that the 3 tagged forum staff members didn’t weigh in, not even on the request to pin the topic 
@Marrelleb could we pin this topic globally for a bit to collect more responses on the poll?