The following is for characters who haven't been confirmed yet, but should still be taken into consideration...
Rosalina vs. Wolf: Wolf has a couple ways to get to Rosalina without worrying too much about the Luma's presence. His f-smash is not only an attack, but also an approach method, and once he slips past the Luma, it'll come down to attack priority. However, trying to f-smash Rosalina could cause Wolf to get punished if she blocks all the hits. Alternatively, the Fire Wolf enables Wolf to not only get past the Luma without much trouble, but because it scores multiple hits, Rosalina can't simply shield herself to try to punish that move.
Rosalina vs. Captain Falcon: Against Rosalina, Captain Falcon does run into a good deal of problems. While Captain Falcon may have the high mobility, his slow attack speed could prove to be his downfall against Rosalina's faster attacks. Add the fact that he hardly has any safe approach methods (two of which can easily be punished with shield grabs), and that can potentially result in a checkmate for the F-Zero Pilot. Lastly, Captain Falcon may be in serious trouble if he's trying to recover from a KO, since the Falcon Dive can't grab the Luma, leaving him completely helpless against Rosalina's edge-guarding maneuvers.
Rosalina vs. Jigglypuff: Attack range is the huge subject here, as even though Jigglypuff's aerial mobility is very good, its reach is not. As such, Rosalina can easily counter most of Jigglypuff's offensive approaches with her wide-ranging attacks. Trying to punish Rollout, however, could be tricky for Rosalina, due to how fast-moving it can be.
Rosalina vs. Lucario: This matchup has some double-edged sword moments, considering Lucario's aura mechanics. At low damage, Lucario may not be able to give Rosalina much pressure, but if Lucario's damage gets built up, even with the Luma, Rosalina will have to be very weary of its increased attack power. The main issue working against Lucario, however, is that it actually has to be trailing in order to truly give Rosalina a lot of pressure, which can make it rather difficult for it to maintain a good lead, if it has one. As a result, this kind of matchup could go either way; Rosalina would have the upper hand if Lucario leads, while it's the other way around if Lucario is trailing.
Rosalina vs. Ice Climbers: This matchup has a lot of questionable scenarios, especially when you bring the Ice Climbers' chain grab maneuvers into the mix. If anything, the Luma is pretty much the main thing that can potentially save Rosalina from the Ice Climbers' grab game. But considering that the Ice Climbers' grab range is rather poor, simply trying to grab Rosalina could be a challenge if Rosalina's d-smash has a longer range than the Ice Climbers' dash grab. If the Ice Climbers are to have any chance of chain grabbing Rosalina to death, they need to first take care of the Luma. However, even if the Luma gets taken out of the game, if a new Luma can be summoned instantaneously, AND can be performed, regardless of Rosalina's actions, this could make Rosalina one of the very few lightweight characters whom the Ice Climbers can't properly pull off chain grabs against.