I can surmise that we will get a brand new edition of the film, but it may not be in time for DVD.
Disney has this new concerted effort after the release of Meet the Robinsons to put out "OverLooked Treasures" into 3D and rerelease them to theatres. The Great Mouse Detective being one of them (filmick and jimhillmedia had this story, you'll have to search for it on the sites because its a little dated).
I would imagine they would put out a new DVD for this, but it might be in the next generation with a bunch of special features and such to promote the next generation. DVD had Platinum series, HDDVD/BluRay will have Overlooked Treasures or whatever.
Or they could skip 3D rerelease and go straight for the collectors market again. I'm banking that they will put out a 2 disc DVD of TGMD next year because this 3D digital thing just won't happen for old movies, but they will have remastered it, so why not release it. That's all just a smige of Speculation, and getting the Gold Editionright now will be fine if you haven't wtched it in a while.