I guess my only two concerns are...
- The difference in layout (the main reason using "wikis" is so popular is they share a common layout that's easy to understand and use).
- The difference in back-end code (Wikis are easy to edit because you can go out and find example code for just about anything you'd want to do).
Despite being one of the editors of the original FIRSTwiki, I'm personally not familiar enough with GitHub editing to be able to contribute in any meaningful way at this point. If you want more help from editors like me, you need to work on two major things: Page templates (basically blank pages with the layout structure there, to allow for "fill in the blank" use) and example pages (a few pages of each type that are all, or mostly, filled in, so editors know what completed pages should look like).
Also, I'd be curious if Brandon made a backup of the original FIRSTwiki before taking it down, getting a copy of it from him could drastically speed up adding content to the site and help fill in details from more distant history that many of us either weren't around for, or don't remember well.