I simply don't think Rosalina will be a character that could be dominated, and in general, they seem to be handling the balancing pretty well in this game. We won't know for sure until the game comes out, but I feel like everyone is assuming Rosalina herself is going to be weak, when we've got more reason to believe otherwise. Luma is obviously the reason people assume that, but think - which is going to be the STRONGER one - the little star child or the space deity?
Plus, the control over Luma is not direct, and positioning him will take some skill. So Rosalina is going to have some power to make up for that, plus her melee attacks have exceptional range due to the cosmic effects that seem to form the hit boxes of her attacks. I really think the "big, easy target that is very light" is Rosalina's ONLY major draw back, and it's a big enough one to prevent her from being OP, I feel. I don't feel Luma is there to make up for Rosalina lacking attack strength or knock back, he's there to draw attention away from Rosalina and be a nuisance to her opponents, making it harder to get hits in on an otherwise easy target. I think the added combo potential is more of a secondary thing. I think not every Rosalina player will be great at utilising Rosalina and Luma for tag team combos, but those who aren't will probably focus on using Luma as a decoy to let Rosalina approach easier. So essentially, we will probably have offensive Rosalina players and defensive Rosalina players, and both will be equally valid playstyles. I think this is another way in which the dev team is bridging the gap between Brawl and Melee.