So there was a pretty great thread about character placement on the character roster on reddit's smash bros section and it pointed out two things I missed:
- The families of characters are in descending order from oldest to newest
- All of the characters with one franchise rep are in order from oldest to newest starting with G&W and ending with Shulk.
Now the trends I noticed still seem to hold true with characters not moving around, but I think the chronological order part is a lot sturdier of a constraint. If you want to suspend the characters 'can't move around thing', basically every single franchise can introduce two or more new characters except for Mario with the free floating one rep per franchise guys flowing across the board to fill in the gaps. I guess that's one way for you to get just about whoever you want out of the last five roster slots (or four if you want to follow the final gematsu character).
As for me, this would be my hopeful roster using the last slots arranged in the style I just mentioned (and yes it includes the big three):