I would omit Ghirahim, Matthew, and Mike Jones (StarTropics) from that list.
However, yeah, Ridley, Little Mac, and Miis are pretty much the only major Nintendo characters left that are not yet playable in Smash Bros. Toad, K. Rool, and Dixie Kong are notable to a lesser extent. There also exists a lot of requests to bring back Mewtwo and Roy. Other than this, Brawl pretty much got in nearly every notable Nintendo character.
Miyamoto's new IP (if it does have a character that could be made playable) is an obvious choice too.
Personally, I don't think Smash 4 needs as much newcomers as Melee and Brawl did. We are scraping the bottom of the barrel now.
Yep, besides any except for the few big ones remaining can be added via DLC if there is enough demand.