Do we have any evidence that Metroid Prime 4 is coming next year? As far as I'm concerned, Pokemon is their big Switch title for next year.
The problem with Sylux, though, is that all the talk of him being in Smash is due to the promise of the character, not the character itself. So far, he's only had one not-very-prominent game appearance, and two teases. He hasn't actually played a significant role in any game or media yet.
The reason people talk about him is because he will appear again, and he will be prominent. But there's no hard confirmation of any of that yet, and it wouldn't be wise to put in a character whose big appearance hasn't even happened yet, especially when there's so much that can change between now and that appearance. His design, his powers, his story, all of that isn't final until the game is gold, which Metroid Prime 4 is far from.
Speaking of his design, it's... not great at the moment. He only has two textures: shiny blue and glowy green. That works for a DS game, but for Super Smash, that's going to look ugly. I imagine his design will be very different the next time he appears.
I know he'll come back in Metroid Prime 4, and I hope it's awesome and that he brings something new and interesting to the series. At that point, yes, we should put him in Smash. But there are plenty of other characters from games that are already out that are worthy of entry. Putting one in who hasn't earned it in any way doesn't make sense.