A Troll in Central Park/WMG

Stanley's "magic" is really Spiral Energy
Consider this: it's colored green, not to mention the whole "it'll happen if you believe hard enough". Which means if he isn't stopped, he'll cause Spiral Nemesis.

Gnorga is a bad troll.
A very bad troll.

Stanley is actually Torok the Troll
Or possibly related to Torok. He somehow ended up in a land inhabited by stupid trolls, and plotted his way into escaping and taking over the world with a thumb that generates plants, succeeding this time.
 * He's his inbred grandson.

Stanley is a bad troll.
Seriously. He's a depraved megalomaniac.

The very bad troll is the hero.
Stanley is, as previously established, a bad troll. The troll who sings "I'M A BAAAD TROOOLL" is the one who first suspects him, and tried to catch him in the act of growing flowers. See, trolls who grow flowers almost always destoy cities by covering them in shrubbery. This troll is the one in charge of keeping rogue flower growers in check.

Gnorga is too psychopathic to be the hero, as she is willing to kill for her goals. Her husband is also a flower grower, and is in league with Stanley. Rosie and Gus are their pawns, which is why Stanley is sent to Central Park. The parents were in charge of allowing the kids to escape to the park, and then bring them back to revive Stanley if Gnorga succeeds. Maria is just an idiot.

Therefore, we should side with the "Bad" Trolls and their Anti-Hero Antagonist Gnorga.

A film-hating wizard created a curse to switch the endings of Little Shop of Horrors and A Troll in Central Park.
Of course this doesn't really make A Troll in Central Park any better, as the true ending involves Stanley almost losing to Gnorga after an awesome Villain Song, only for Gnorga to be electrocuted to death by a stray wire, and Stanley and Rosie go and live in the suburbs somewhere.
 * No, no, because to do that the one who dies would have to be the one who covers New York in plant life. The true ending, then, would involve Gnorga almost losing to Stanley after an awesome Villain Song, only for Stanley to be electrocuted to death by a stray wire.

The film is a Future Imperfect retelling of the events of Little Shop of Horrors.
The characters themselves were wiped out by the plantocalypse, and when the Audrey IIs left the Earth they only took their pods, leaving immobile vines. A Troll in Central Park is an attempt by the survivors to explain what happened, years later, although they have somewhat forgotten of the violence of the plantocalypse.

King Llort is secretly gay for Stanley.
King Llort is just like Stanley, only he doesn't have the green thumb that would allow him to grow flowers. The reason why he is married to Gnorga is because he was actually part of another troll colony that was made up of nice trolls, and his kingdom wanted to make peace with the bad trolls. And so, there was an Arranged Marriage. Stanley may or may not have been part of the nice troll colonies. This explains why he went out of his way to make sure Gnorga didn't turn Stanley into stone.

A Troll in Central Park is the unofficial Troll 4
Though this one actually has trolls.

Idea that Stanley might be a True Fae...

 * Alternate Character Interpretation- It is possible to see Stanley as not so benevolent troll, considering he dreams of a land where all trolls are essentially copies of him (even if supposed to be "good") and, in the end, . Whether he is simply delusional or actively malevolent is up to the viewer.
 * Here I thought he was just a True Fae...
 * OH GOD. Do you realize how horrifyingly terrifying this makes the movie!?!

The movie is actually seen through the eyes of the minor squirrel character who looks high.

 * The actual plot would have made a lot more sense if not seen through the eyes of that stoner squirrel. Everything's from his point of view.