Great work guys! Looking forward to the update.
About that forum though.
I can understand the reluctance to modify the forum because, even as sprawling as it is, I'm guessing it's one of the biggest "stickiness" factors for the website as is.
One approach that might be the most painless is to leave the basic structure as is, but make it much easier to find threads that allow users to discuss what they want to discuss. This could be a two step procedure:
1) allow forum users/mods to tag threads with a number of pre-defined tags in a set number of categories (see below).
2) create a universal sort function at the top of the main thread page (or directly below the L2 section threads) that will allow users to filter for certain tags.
Well the way I figure it, there's a relatively few (4?) themes with which you could categorise the threads in the forum to make it much easier to navigate:
1) By destination object
(choose object Moon, Mars, Jupiter, all planetary bodies, etc) or region
(LEO, GEO, LLO, SEL2, all orbital regions etc), or leave undefined.
2) By space agency
(choose NASA, ESA, JAXA, all government agencies etc), or space company
(SpaceX, ULA, Blue Origin, or all private companies, etc) or leave undefined.
3) By scientific objective
(astrobiology, solar system/stellar observations, human space flight health, all scientific objectives, etc) or commercial objective
(telecommunications, space tourism, asteroid mining, all commercial objectives, etc) or leave undefined.
4) By time period
(for real-life historical, current day activities/news, future within planning horizon, far future, all time periods) or popular culture relevance
(for polls, discussion of science fiction, and other uncategorised/uncategorisable but relevant material), or leave undefined.
So beside each thread would be an option to "tag this thread" which would give you a dialogue box with a set of 4 or 8 drop-downs: each of which contains a standard set of tags (one of which is a catch-all type tag) that the user base can go through the hundreds of threads in the forum and categorise all threads with relevant tags.
Then, using the filters on the main page is basically an advanced search function, showing you lists of threads as you would see within each section, but tagged with the tags you chose in the filters.
Hopefully makes sense... Good luck however you choose to tackle it.