Knuckles being shown as an Assist Trophy while Shadow was MIA and the image with Rathalos that from what I heard appeared to reference Golden Sun.What do you mean by 'hints'?
I put "hints" in quotes since nether of these were direct, foolproof "hints" but they did lead their fans to speculate.
This argument hinges on everyone agreeing that these characters shouldn't be playable, despite the fact that plenty of the characters you mentioned have supporters of their own.You know how people argue that Goku would open the door to a lot of non-video game crap?
It's the same thing with spinoff characters. No one would want Lana, Linkle, Rowan, Shareena, Fawful, or the Paper Mario partners. I understand certain spinoff characters are popular, but rules should be rules.
The difference between non-video game characters and spin-off characters is that Sakurai himself said, in no uncertain terms, that non-video game players wouldn't be coming to Smash. Absolutely nothing has been said against spin-off characters. This is the prime example of those "fan rules" everyone loves so much.