Truth be told, we're nearly out of first-party 8-bit era characters that could really bring something to the table. Many of the characters passed around like Balloon Fighter, Excite Bike, Mach Rider, etc all are one-trick-ponies that would require a severe amount of made up abilities to make them be able to do anything other than their one ability in their own games.
Then compound that with the knowledge that there will be less newcomers overall this time around. It makes going through all that trouble to bring back a character from a series that hasn't seen much, if any, action for some decades, and thus lower exposure fans, seem like a lower priority. Remember, Takamaru was explicitly left out of Smash 4 because he was too obscure, and things aren't any better for him in Ultimate. I'd imagine as time goes on, many of those other less-known NES characters will, or already have, suffer this fate also.
I'm sure we'll get some out-of-the-blue surprise character, but I don't think it will necessarily be from so long ago.