More a dream roster than a realistic one, especially since I doubt anyone on that bottom has a real shot with being in nor do I think the real roster would have more than 45 slots. That said, I think this would be quite to a perfect roster imo, one can only hope. Will put a another roster up when I got the time to look up better pics and decide on some replacements.
If you're looking for tips to make this realistic:
-4 Star Fox characters is pushing it, as is 6 Mario characters.Consider removing Krystal and/or Bowser Jr.
-Remove Blaziken. Sorry, but he just isn't happening.
-I'd remove Dark Samus as well. DS isn't as unlikely as some people might suggest(being the main antagonist of the Prime sub-series and being in as many games as MB), but s/he's still a somewhat out there choice. And of course, Ridley would come first.
-On the bottom row: Little Mac, Isaac, Shulk, and Pac-man can stay. As for the others, remove all but one of Chibi-Robo, Wonder Red, Balloon Fighter, and Ray. Having any one of those characters probably wouldn't be such a big deal, but having all of them is pushing it. Dillon is plausible IMO, so it's up to you what to do with him.
Now as for who to consider adding:
-consider adding Takamaru.
....and that's it.
For a "Dream Roster", this actually isn't too far out there. Good first try.