It's always nice to hear about someone writing a cool program just because they felt like doing so. Every time I think about writing a forum server, I find I return to the thought that Usenet was better, and that if I wanted a web forum, I'd consider putting a web front end on an NNTP server. That would among other things let people keep using NNTP clients (I like gnus.el) if they don't want to use a browser. There are a bunch of NNTP web clients already, but afaict they all needlessly suck.

You might take a look at Fossil (fossil-scm.org) which is not a forum, but is an all-in-one source control system, bug tracker, wiki, and web backend, all implemented as an SQLite client written in C. Its blurb says a scripting language might sound more attractive than C superficially, but in Fossil, SQLite does all the heavy lifting that would otherwise have been done with scripts. On my bottomless todo list I want to study its implementation sometime, since I think it would be a good example of "advanced" uses of SQL and SQLite. That is completely independent of Fossil's purpose as a VCS application. I'm ok at basic SQL but not so clueful about sophisticated uses.

One thing to watch out for is that since SQLite is an embedded DB, when you make an SQlite query, the overhead is basically a local subroutine call rather than the network traffic that a client/server DB would incur. That in a way changes the asymptotics of an application: particularly, the "N+1 queries" antipattern of some client/server DB apps is a perfectly ok design strategy for SQLite. I believe Fossil may rely on this in some way.