I think a 6th mario rep is possible and I think it will either be bowser jr or paper mario but (and there's a little biais in that) I think paper mario has a little more chance than jr here's why:
Both of the characters would be added because of their uniqueness. That's how I see them I see their uniqueness as one of the main reason to worth a 6th rep. However, paper mario has an advantage that is quite unusual: since he's from a different series, he has more potential than what you would actually expect. I see alot of people claiming that it would be the toon link situation all over again but it goes ALOT further than this.
Toon link has one thing in common with him: he's the same main character but represented differently. The main inconvenient with toon link is that if you look at the whole character, even if you try, he would still feel like link which is why he ended up being a clone. This is due to the fact that not only he is link but also because he come from the same series as link which also means similar abilities.
Here, this is not the case. Paper mario is mario but is from an rpg series while the main series is a platformer. Therefore, to suit the character, he HAS to have a playstyle not suited for platformer abilities but rather rpg abilities. But it gets even better: because of this, it has a so big potential in like every department that he is more unique than enough. There's a reason why I think he has "infinite potential" go to the thread to check for yourself it's crazy. There's even potential to have gimmicks and you know how Sakurai might like those.
Also, may people can claim that the main series is repped properly. However that is not the case with paper mario: with only trophies and stickers, it deserves more than that. To put this in perspective, M&L got one music so why paper mario, with one of the most diverse ost, can't have a single one? So, not only there's a need for uniqueness and big potential but also for the series itself. We know Sakurai likes to rep new series now so why not doing the same for this series? There's a big need for it.
That is why I think his chance are helped a little more than jr. Both are worthy addition and Sakurai will have the final word on it. Also, no paper mario has no advantage by reping his own series instead of the main one because it doesn't matter that he's from the paper mario series and that he would be added because of how big his potential is not because of which series he reped.
Would like to receive feedbacks on this. Remember, my opinion, my interpretation.