Looking at the two movies more closely, the differences become even starker. Gremlins is a classic horror-comedy about cute little creatures called mogwai that, when fed after midnight, become crazed-out, slimy gremlins that want to fuck shit up, and might also be a metaphor for puberty. Little Shop of Horrors, on the other hand, is a musical about a sentient monster plant called Audrey II that wants to take over the world and eat all the humans. Other than the monster angle, there doesn't seem to be that much overlap between those stories ... but it'd be amazing if there was. Imagine it: a whole movie about two supernatural threats to humankind, one chaotic and insane, the other insidious and malevolent, joining forces to spread death and destruction like animatronic versions of the Joker and Bane.
"If I pull off your leaves, will you die?"
"It'd be very painful ... for you."
Why It Could Actually Work:
Now, what I said earlier about the gremlins and Audrey II being "supernatural" threats to humanity? That's not entirely accurate, because both monsters are in fact aliens.
We know as much about Audrey II thanks to the song "Mean Green Mother from Outer Space," which reveals the plant's origin as an alien species "from past the stars and beyond the moon." As for Gremlins, the official novelization of the movie explains that the mogwai were originally created on the planet Enz by an alien scientist called Mogturmen, who easily could also have been the creator of Audrey II.