I'd love to see Klonoa but I HIGHLY doubt it'd happen. #Isaac4Smash
Also, I'm not entirely sure that form of fan service would be why they would add her. She is a very popular character and her games are fun. She has as good of a reason to be in Smash Bros as Shovel Knight.
Another reason Shantae is a popular addition, at least in the Anglosphere, is that Matt Bozon, creator of the series and the lead artist, has been quite actively campaigning for her inclusion into
Super Smash Bros. 4. She is 3rd-party and almost unheard of in Japan, however, so it's unlikely. On the other hand, the same goes for Shovel Knight, and he got an amiibo, so anything could happen.
The general rule for 3rd-party characters in Smash Bros. seems to be that they must be characters who are legendary in video game history and more or less represent the company that makes their series. The only exception so far is Snake, and that's because Hideo Kojima and Masahiro Sakurai are friends. (And at this point, as they are paid DLC characters, that character must be reasonably popular enough to make back the money spent creating that character in DLC sales. Someone like Shantae CAN pull it off, though it's no guarantee like Inklings and Octolings. Someone like Klonoa has no chance. Rayman does, though Ubisoft promoted the heck out of the Rabbids during the 7th generation and pretty much stole the public attention away from him.)