Lots of things could be. Whether they're worth adapting is another question entirely.
Most shounen fighting series are out. The genre thrives on things like stopping the action to explain what's going on every few pages, which the LA audience isn't likely to tolerate.
Mecha - more feasible, given that there's a large tradition of high-tech, effects-laden sci-fi. Good sci-fi, even. Still, for such a series to be at all memorable, it'd have to place much more emphasis on the characters than on the machines (when Star Wars started substituting effects for characterization, it started going downhill). Only something like Transformers, where the robots are the characters, can really get away like that. Hence, a Ghost in the Shell (or a Battle Angel Alita) is more likely to work than a Gundam or a GaoGaiGar.
Romance (either gender) - If the story's good (really good), you might make it work. Alternatively, restructuring something like Toradora into a sitcom, adding more elements to the overall story that aren't related to getting the two main characters together, could work. FrienRAB was very successful and had major plotlines related to male/female relations, but they were far from the only thing the show ever dealt with.
Other - anything focusing on human characters that wouldn't end up being entirely effects-based could work. Monster has LA written all over it, Death Note's already been done, Darker Than Black isn't so different from Heroes in terms of the people in it, Uninhabited Planet Survive is Lost in Space with a lot less cheese, Daughter of Twenty Faces, Dennou Coil (wait, 12-year-old lesbians, never mind), etc etc etc...
The biggest issue here is probably that anime fans are willing to tolerate a lot of things that live-action viewers don't, so much. Things like shounen's constant pausing the action for commentary, mecha's tendency to let the effects replace the characters, constant recycling of the same romantic-comedy plotline in every episode, shows that take months to give you any sense of the characters... They're held to different standarRAB.