I'm surprised I didn't see this thread sooner. I think the reasoning behind your rosters makes sense.
Just an guess, but newcomer franchises like Wii Fit and Animal Crossing will probably only have one character. Franchises previously in Smash have a better chance of having multiple characters.
I thought of something. If you draw a triple venn diagram of SSB, SSBM, and SSBB's rosters, any character that overlaps with another game's roster is very likely to return. For example, Yoshi appears in every game, so he's making a return. Mewtwo only shows up in SSBM, so he is unlikely to return. Ice Climbers show up in SSBM and SSBB, so I think they have a good chance to return with Ganondorf. Just a theory, but it's interesting. I might make a visual in photoshop if I have the time. lol