Sirius no Densetsu

Both are old classics about an impossible love.  

Both films are similar in themes of unlikely love, characters from different worlds meeting in love in conflict, and similar character design. 

Both films are adeptly written stories that are extremely well executed and are guaranteed to make you cry. A beautiful love story, similar to the tragedy of Romeo & Juliet (but FAR BETTER!), this film has the film quality of Studio Ghibli and a soundtrack composed an orchestrated by the Tokyo Philharmonic Orchestra. It is so beautiful that this music will haunt you for the rest of your life. I highly HIGHLY recommend this one.  

They both share the theme of love that is forbidden or looked down upon. 

Even though Romeo and Juliet is not a movie it still has the same storyline. The Sea Prince and the Fire Child is about two creatures from different worlds who are suppose to despise each other. They then meet and fall in love. Same goes for Romeo and Juliet they are two lovers from families who are enemies. Both of these animes end the same way. They also have other similar features. For the most part it is the storyline.