Maybe consider DK Jr. as a possible historical character? He fits the type of archetype Duck Hunt did (massively well known and popular game that no one expects a character from due to age) and would be an interesting way to represent the classic arcade origins of DK and Mario, which have only gotten more referenced as of late.
Additionally, I'd definitely recommend considering Kamek as a newcomer that people are sleeping on. He's the main antagonist of a game series that had three releases between the end of Sm4sh roster selection and the beginning of Smash 5's roster selection and which is going to receive a Switch title soon (this year?) He's also extremely recognizable, arguably being the tertiary villain of the massive Mario franchise. Further, his magical abilities allow for a lot of flexibility in moveset options, to the point that him summoning and buffing enemies to attack the opponent is a real possibility. He's not particularly popular a choice, but he fits the bill of a Rosalina/Bowser Jr./Robin-esque choice that makes sense in hindsight.