Funny you ask, I'm actually in the process of learning The Theme of Marco and Gina on the piano, lol!
Thoughts, though? Porco Rosso isn't my favorite Ghibli film by far (Princess Mononoke will forever be my #1), but that doesn't mean it doesn't hold a special place in my heart for nostalgic purposes. I'm a sucker for magical realism in general so you can imagine why I'm kind of a Ghibli superfan. It's particularly prevalent in both Porco Rosso and Whisper of the Heart; I like how Marco being a pig is symbolic of his survivor's guilt, but I do think it could've been taken to another level had there been some exposition at some point in the film explaining how and when he morphed into a pig---nothing too crazy, I just want to know: did he wake up one morning, changed overnight? Or was he in the sky, in his plane? I really think it could've solidified an otherwise ambiguous part of the movie that I remember, as a kid, thinking was confusing. Maybe the ambiguity was intentional---actually, it likely was.
Either way, watching it once I could draw a clearer conclusion was cool, something I've experienced with each Ghibli film since I watched most of them when my brain was small and stupid, only to later rewatch them from an older perspective. I've seen criticism for the open-ending in Porco Rosso as well, and I guess I agree, though it doesn't bother me all that much. Again, I like Marco as a pig and I'm glad we never see what he looks like as a human.
That was a little longwinded, sorry! Just my two cents on it!