I see 3rd partys as having to be incredibly iconic characters who carry game history (Sonic, Cloud, Snake, Pac Man), so close to Nintendo that they are practically Nintendo or were part of Nintendo (Bayonetta and Banjo/Kazooie), or both (Mega Man, Ryu, Simon).
Shantae began on a Nintendo console and was exclusive to them for 3/4 games. Also unlike Shovel Knight, Shantae has had a long history with Nintendo and has 4 games, so she is an established franchise. Not to mention the character has moveset potential. If any indie character is deserving it is her. Indie characters shouldn't be shoved aside just because indies and not very mainstream, they should be judged on the merits of the character. Plus I think it would cool to have indie in Smash, showing that even smaller video game studio's can get a character into Smash if they are deserving
Since you mentioned Spyro and Rayman, I think they are less deserving than Shantae as they are not that iconic (at least not to the level of Pac Man, Metal Gear, Sonic, Street Fighter, Mega Man, Castelvania and Final Fantasy) and are not associated with Nintendo that much. Yeah they have had games on Nintendo, but people associate Spyro with Sony and Rayman has been a multiplat, not really associated with any one console. Mega Man and Castelvania were very associated with Nintendo during the NES/SNES era, Banjo and Kazooie was the face of Rare during their days on the 64 and is their most popular character (not to mention they were owned by Nintendo and the N64 wouldn't even be half as good without Rare). Bayonetta didn't start off on Nintendo, but they revived the series and the 2nd and 3rd games are exclusive to Nintendo since they funded them, so she is like Nintendo's adopted child . Not saying Spyro and Rayman don't deserve to be in Smash, I just think Shantae has more merits than they do.
Yeah Isaac and Geno are safe picks, but I don't consider Banjo/Kazooie safe picks at all. Yeah they were the most popular third party character in the ballot next Snake and the head of Xbox said he was okay with letting them in Smash. Even though Spencer's comment made Banjo become a possibility, it is still a giant hurdle getting a character from a rival console and has never been done before in Smash. Maybe Nintendo wouldn't like working with them or Microsoft asked too much.
Ultimate's theme in terms of characters has primarily been fan service, Gen 7 wasn't announced until a couple month after the ballot ended and Sakurai seems to have more interest in adding highly requested characters for the most part.
Lol, just take it easy man. It hasn't been confirmed or deconfirmed yet. The only way we will know if it is true the next character that gets revealed is one on that poster and they are in the same spot in the poster as they are in the leak, similar to what happened to Shulk in the ESRB leak.