I don't think that the guts of DiscussionPlugin are so awful that it would be better to scrap it entirely and start over. API.py, model.py etc. have some deprecated DB API structures and such and need some updates to go along with changes in Account Manager etc., but are mostly sound in terms of code and data structures. They mostly import core Trac libraries, so migrating to Python3 along with Trac itself shouldn't be a bear. Some
The templates/CSS/JS OTOH could use some major work. They're all still Genshi, and thus need Jinja2 replacements to move forward. They're also ugly from a UI/UX standpoint. The CSS overrides some of the underlying styling, but not all, so you can get some unfortunate light text on light backgrounds and things very easily if you've got a darker main theme. That's an opportunity for greater return on effort than writing a whole new discussion plugin.
A PhpBB/XenForo plugin that exposes inter-trac links in BB code might be useful apart from that, as no add-on for Trac is ever going to be as robust as a dedicated forum system. Something like [tl=trac-hacks:#756]some ticket[/trac] might be useful over regular [url] links if the plugin config could define inter-trac aliases, along with maybe a default one if none were specified. That's a PHP job though.
Best,
--Jonathan Laufersweiler