There are people who hope some of the darker series would come over with a serious translation. Thus far only Ultraman and Ultraman Tiga (out of print now) has gotten it. To be fair it's not like Ultraman is super-popular in the U.S. but still...
Ultraman: the Jupiter Effect, penned by Jeff Segal in 1978: The plot has the alignment of all the planets in our Solar System cause major disasters around the world, causing monsters to invade. Ultraman (who's host would have been a NASA astronaut) appears to save the planet. Things would have gotten so tough that the other Ultramen from the franchise show up to lend a hand.
Ultraman: Hero From The Stars, penned by book writer Don Glut in 1983: The plot would have involved highly evolved dinosaurs attacking the Earth after years of waiting, and a new Ultraman inhabits the body of a Earth Defense Soldier to fight back. Notably, this script featured the death of Ultra Seven during the film's climax. The screenplay was said to be pretty bad however, and it's said it ripped off too many cues from Superman (At one point of the story, the new Ultraman even tries to impress his girlfriend with his gigantic super-powers!).
Ultraman was originally going to be in an Ultra Q episode, where he was going to be an alien villain, instead of a superhero.
Ultraman was originally going to be a bird like alien monster named Bemlar. Due to the fear that children wouldn't know who to cheer for, they turned Ultraman into a more humanoid alien. The name for the Bird man was given to Bemlar while it's design would be sold to Toho and be incorporated into the poular monster Rodan.