I like you.
Though in regards to the last point you made about new franchises without any hints, I think something that needs to be considered is how they could actually be hinted at. If you look at the other characters you listed, those characters were teased and hinted at, but those hints and teases were never exactly huge, obvious hints. For example, the Mario Galaxy and Lumiose City stages, while hints towards Rosalina and Greninja, respectively, weren't blatantly obvious hints towards their inclusion, as the stages themselves were pretty logical and expected choices. Similarly, Ridley's teasing looks like (and may actually be) a de-confirmation; if Ridley ends up being confirmed, it would be clear that he was being hinted at all this time, but it wasn't to the point where his inclusion was guaranteed. Far from it. Now, in regards to series like Xenoblade, let's say that in a random pic of the day, they revealed a Xenoblade stage. I would imagine that most people would interpret that as an obvious clue that Shulk was going to be a playable character. After all, why bother adding in Xenoblade content like that without the main character? In a case like that, I think the hint would be too obvious. The closest thing we've had to a situation like that is with Little Mac and the Boxing Ring, and look what they did there: they completely re-skinned the stage to hide the fact that it was a Punch Out stage. If they hadn't done that, Little Mac's inclusion would almost be completely given away by such an obvious hint.
So basically, I think the reason why certain series with potential characters haven't been teased yet, like Xenoblade or Golden Sun, is because, frankly, any teasing regarding those characters would be too obvious. Well, there's also the fact that a lot of those series are not going to get playable characters, and thus won't be teased at all, but since there are certain series (Xenoblade) that seem very likely to get a character at this point, I think the reason for a lack of teasing would be because any teasing at all would just give away the surprise.