I don't know how relevant this thread is anymore, but since it has not been locked I assume it's still viable.
That said, I would love to see the expansion of BBCode. A big thing I'm missing is tables, I think it would be a lot easier to do special formatting on posts and signatures with BBCode tables, and other BBCode, since we don't have the ability to do raw HTML. It should be fairly easy to implement in the Admin CP, an d I believe there is a plugin for it as as well.
Even better would be the complete overhaul of BBCode by replacing it with markdown. This is more well known at this point probably than raw BBCode, since it's widely used even in this community, with things like Discord and Showdown, and it's also widely used with sites like Reddit and Github. It might be possible to set markdown in the Admin CP fairly easily, or it might require some backend stuff and changing the parser in the PHP files for BBCode... either way I'm willing to bet there's a plugin to add it.