I like this idea a lot and I am still sorting the implications which could be revolutionary if this can be done.  This could be huge. 

Random thoughts from the top of my head: 

This could be a big boost for non- Big Tech search engines, niche search engines like Marginalia Search, Indieweb Search, and various web directories. Plus, as mentioned, wikis like Indieweb.org. 

I always have the notion that we are underutilizing both web search and site search on the web.It strikes me that having this as a browser extension is on the right track although it might be easier to make this platform specific like plugins for WordPress.“downsides and social ills” 

Right now I can’t think of any but they will exist because this is the Internet and anything that can be exploited will be exploited.Searching one specified off-site source with a wikilink may not be too hard but trying to do a meta search of several sources could be very difficult unless each source searched opens in a new tab.The UI will be tricky. If you want wide spread adoption then it needs to be simple. 

Even DDG’s !bangs are a bit too complicated for mass use. 

Anyway it sounds like something to be discussed, thought about and experimented with.