When you think of it, a forum is really limited in functionality.
About 90% of the user interfaces are only accessible by the admin.
About 95% of the user interfaces are only accessible by the mods.
If you did the ‘math’, you guys (‘regular members’) see about 5% of SMF’s UI, which is the part that initially has to work in the revamped forum.
When you think of it, the UBB parser (responsible for turning your [icode][i][/icode] into [icode][/icode]) is the only complex thing you’ll encounter, as UBB is quite a messy standard, given that an AST has to be built with different parsing-rules per tag-type. It also has to be able to process garbage and gracefully continue parsing. The good news is that that part is done already. SoonTM we will have (near) realtime-previews of the post you are typing.
The data model is simple, there are only 20-30 tables, which have simple relationships. The complex part is really the templating and the separation of concerns of serverside vs. clientside). With modern clientside templating engines like Angular / Knockout / React, things become exponentially simpler.
With control over the backend implementation, it’s actually quite easy to make most of today’s forums functionality less… stupid. Once you follow a topic or participate in it, it should take priority in the unread-topics listings, say: Followed > Participated (ordered by, say, awarded medals, desc) > [everything else]. Things like infini-scroll become trivial.
One of the more annoying things to support will be handling all those arcane URLs that are indexed in search-engines already.