I think I have an idea who might be a logical choice after Mr. Game & Watch, R.O.B., and Duck Hunt. Let's see...
Mr. Game & Watch reps the Game & Watch games.
R.O.B. reps the NES Robot peripheral and games.
Duck Hunt reps the NES Zapper peripheral and games.
How about someone who represents the NES Power Pad peripheral and games?
No, really... I think this could be feasible... even "realizable". It is not like Nintendo forgot about the NES Power Pad. Check this out.
See? Reggie playing with the NES Power Pad in 2015.
Anyway, the game that came bundled with the NES Power Pad is World Class Track Meet. It was also bundled with Super Mario Bros. and Duck Hunt.
The character who represents the NES Power Pad could be named "Track Meet". He looks like your generic track & field athlete except he is always on the NES Power Pad. He stands on it. He runs in place on it and it moves along the ground. It disappears when he jumps and reappears when he lands. It floats like a flying carpet as a recovery special move. He might even have a "jumping over hurdles" special move where a hurdle pops out of thin air and he jumps over it... and the hurdle sticks around for a while to block projectiles and forces opponents to jump over it (it won't let them pass). I imagine he would be a fast character with jumping prowess due to the fact he is an Olympic-level runner who competes in long jumps and hurdle jumps.
Please take note at the bottom of the 3-in-1 box art... see the three symbols and what they represent.
Action Series for Super Mario Bros.
Light Gun Series for Duck Hunt.
Sports Series for World Class Track Meet.
Now, where have we seen the "Sports Series" symbol before?
We still do not have a character who represents those NES sports games. Those games were also represented in NES Remix games so it is not like Nintendo completely forgot about them. Track Meet could possibly represent both the NES Power Pad games AND the Sports Series games. Track Meet could possibly use tennis rackets, baseball bats, and golf clubs in his normal moveset in addition to his NES Power Pad special moves.
He might even use a special move or two from the other NES Power Pad game, Short Order + Eggsplode.
Oh, yeah! In the world of Smash Bros., the NES Power Pad is some sort of advanced technology that transports Track Meet around when he runs in place on it and makes sports equipment, hurdles, and bombs appear out of thin air for him to use!
And you have a recipe for a quirky retro character.
In conclusion, Track Meet for Smash 5!