At this point, I think there is no doubt that Rosalina will be competitively viable. The facts are there. We know how she functions. And I can't see them changing anything about her in the final build (apart from any universal changes that effect all characters, like adding wave dashing or something, which could only make her more fun to use), since her reception has been overwhelmingly positive. She won't be buffed since people seem satisfied with her, especially well-known community members like Prog and D1, and she won't be nerfed since very few people could even get to a competent level with her to begin with, since not many people got more than a few matches in the game - and the only reason for this is because she's the most technical character in the game so far, so only people who really got a lot of time with her have truly valid opinions on how she plays, and those people reckon she is fantastic.
Rosalina has weak spots, but I doubt they will be a bother for the best players:
She's one of the largest characters in the game, making her an easier target - so learn how to use that amazing dodge rolls of hers (for reference, her dodge roll looks better than Sonic and Meta Knight's did in Brawl, and they were considered to have some of the best in the game - Rosalina's dodge roll could potentially be the best yet in a Smash game, with it's amazing speed, distance, and "warp" animation adding an element of visual trickery). And learn how to use Luma to negate attacks if necessary.
Luma can get knocked off pretty easily - but only if you're careless, or rely on him too much. Rosalina can fight just fine, so be sure to switch things up between primarily fighting as Luma, and primarily fighting as Rosalina. And remember that you can overwhelm opponents by smartly positioning BOTH Rosalina & Luma, especially in 1v1 settings.
Really, those two points are the only weaknesses that might serve any sort of consistent problem. SoRo may have lackluster K.O potential, but it's a fair design choice, and isn't too big a deal anyway, especially since I was a Sonic main in Brawl. And unlike Sonic in Brawl, that low K.O potential is a brief slap on the wrist for losing Luma, rather than a constant pain to deal with. When Luma is present, Rosalina has monstrous power. So yeah, I'm not worried about that.
She just seems like she has too many possible applications to ever be anything but high tier, IMO. She might even end up as this game's Fox or Meta Knight if people get good enough with her, and then show those skills off in tournaments, thus convincing people who were on the fence about her to pick her up.