Knuckle Joe really does not need to be in Smash (even though he was made for fighting) just because he is an assist trophy. In fact, most of these characters can be considered to being equal to the Pokeball Pokemon characters in terms of being playable (sorry I had to bring this out, but it seems to me that quite a few people believe most Assist Trophies have a chance of being playable in SSB4).
Anyways, Knuckle Joe is just another enemy in the Kirby series, so there is not much going on for him to make him as important as the three we already have in Brawl. I think that if we were to get a fourth Kirby rep it would Waddle Dee (aka Bandana Dee) from Kirby Super Star and Kirby Returns to Dreamland. He has potential to being unique with his spear and waddle dee has been playable a few times in the series.
Aside from that, I'd say that Little Mac, Issac, Starfy, and possibly Tingle and Samurai Goroh have good/solid chances. The first three can rep their own series as no matter how small their franchises may be, we are lacking the number of actual nintendo all-stars to being playable in SSB4. Tingle? To be honest, I think his chances have diminished in recent time (seeing how he was removed in the latest Zelda games); however, he is capable of repping not only the Zelda series, but also his own series (these games were not realeased in America only). Samurai Goroh has his chance; however, is F-Zero deserving of another rep to Nintendo's eyes (even though Mother/EB got another rep)? We'll have to wait and see...