I know I keep banging on along similar lines, but CJ Freezer drew a lot of plans for a nominal 8'x6' garden shed (layout area of 7'6"x5'6"), any of which would benefit from stretching to 8'x8'. Many are main-line plus branch arrangements which would give you the sort of operating potential you appear to be after. If you can get hold of copies of Track Plans and 60 Plans for Small Railways they should both give you plenty of ideas for what is possible. 60 Plans ran to several editions over the years and the layouts included varied between them, so I'd recommend getting 2 or 3 copies of different vintages if you can. Just beware that some of the earlier plans were drawn in the days when 15" radius r-t-r points were not uncommon and so some juggling may be required to fit everything in with more modern track geometry.
Just for the sake of interest, I'll PM you a rather ambitious 7'x5' that doesn't, AFAIK, appear in any of CJF's books.