It's been a topic of discussion for a while now, so here is the straight info:
Belle can't become a Queen because her husband is the Sovereign Prince of a Principality, NOT of a Kingdom. Much like Prince Albert of Monaco.
Same goes for Prince Eric; he is the Sovereign Prince of his Principality.
Aurora will become Reigning Queen upon the death of King Stephan, and co-Sovereign upon the death of King Hubert. If King Hubert dies before King Stephan, then she becomes Queen Consort to King Phillip first, and co-Sovereign after her father's death. Aurora's and Phillip's firstborn will inherit the thrones of both Kingdoms, at which time, Aurora may abdicate and become Queen Mother to the new King or Queen. If Phillip were to die, she would become a Queen Dowager.
Cinderella will become Queen Consort upon the death of Prince Charming's father.
Snow White is the daughter of a King. When he marries the Evil Queen, his bride becomes a Queen Consort. Upon his death, she becomes a Queen Regent. Upon the death of the Evil Queen, IF Snow White returns to her father's Kingdom, then she would be a Queen Reignant and her husband would be either a Prince Consort or a Royal Consort, depending on his intrinsic title and local custom; he would not be a "king consort" because that title is NEVER used. It is unclear what is the nature of her prince's title; he could be the firstborn son of a King, which would mean that upon the death of his father, he would ascend the throne and become King Reignant, with Snow White becoming his Queen Consort. If she reclaims her father's title, She'd be co-ruler, like Aurora, described above. However, her husband might be a lesser son of a King OR the Sovereign or son of a sovereign prince of a principality, in which case Snow White will never become Queen unless she reclaims her father's title.
Jasmine will never be Queen. Her husband Aladdin will rise to the throne and become a Sultan, and she will gain the title "Sultan's Wife".
Pocahontas will not be Queen of any sort. Powhatan was elected Chief (ergo she is a "real" princess) and neither John Smith nor her eventual husband have any form of royal title.
Mulan will not be a queen either. While Chang is noble-born, he is not a prince of any sort... and neither is she.
Tiana is an interesting case; her husband is the son of a King, so upon his death, Naveen should ascend the throne and become King Reignant (so Tiana would become a Queen Consort), BUT... it is unclear whether he would choose to return to his royal life; in such a case, his brother Ralphie would rise to the throne.
Rapunzel WILL become Queen Reignant, with her husband becoming Royal Consort Eugene.
Merida is the firstborn of a Reigning King, though she has made it clear that she will not marry for political reasons. There are two possibilities upon the death of her father: first, she rises to the throne, becoming Queen Reignant. Queen Elinor would become Queen Dowager. Conversely, Merida might be passed over due to her gender or celibacy, so that one of the triplets could become King (or all three become co-Kings... Heaven help the kingdom...). IF one of the triplets is to become king, but is still too young, Merida (or Elinor) could be named Regent until the triplets reach age of majority; If it is Elinor, she would be Queen Regent. If it is Merida, she could be either Queen Regent or Princess Regent; in either case, after the ascension of the throne by her brother(s), she would revert to her title of Princess.
It's too early to speculate about Frozen.
I hope this has helped those wondering who can or can't be queen.