honestly, I've been thinking Contra has been pretty slept on too. It was a huge iconic series on the NES much like Castlevania or Ninja gaiden (not in Smash of course, but Ryu was still someone that people have speculated over to death unlike Bill Rizer), Contra has had various games between multiple Nintendo and non-Nintendo systems, It has been a pretty influencial series, with how mnay games probably took some inspiration from it, and how it gave birth to the Konami code. Also important to note, is that Contra has been relevant to the Switch with stuff like the COntra collection and that new Contra game released on the system.
While Konami already has alot of content in Smash, the characters in the game they did include or bring back were all in base, not DLC, so if we are going with the idea that "a company only gets 1 character for the pass", then Contra isn't out of the running, much like SE still had Hero in the DLC despite them bringing back Cloud in the base roster.
Not saying Contra is likely or anything, but I wouldn't discount it either.
Speaking of SE, another series I'm surprised hasn't gotten a bit more discussion is the Mana series. Secret of Mana is generally put alongside the SNES greats (SMRPG, FFVI, Chrono Trigger, Earthbound...), and like Contra, Mana has made a presence on the Switch with the Mana collection and the Trials of mana remake in the works right now.
Granted, I don't really expect a Mana character to come in, especially if we do go with the "a company only gets 1 character for the pass" idea (personally I don't know if I'm 100% on board with it, but it's remained true thus far, so...). But hey, if people can still bring up 2B, then why not atleast toss the Mana series in the ring too I guess...