I have to agree here with leaving the categories as is for now. I think adding the Jr category is the right move, but breaking up the others into different scales, or scratchbuilt vs non scratchbuild....at this point it is way too early.
The only thing I could see at this point maybe would be to break up the All Open and subdivide it a little (aircraft, armor, dio, auto, etc, etc). Maybe have a separate sub category for each genre in it, the one in each of the genres with the most votes goes on to compete with the other winners in the All Open category, giving a 1st, 2nd and 3rd.
Otherwise the categories to me are good to go for now, and for awhile I would think. Last thing I would want to see is the contest ending up like an IPMS with 70-80 + categories.