Grasshopper Manufacture's Goichi Suda has told 1UP that No More Heroes 2 would be the last game in the series on Wii, until the next version of the console is released.
"Looking at the Wii, I think No More Heroes 2 will probably be the last iteration for this current platform," he told the site. "But there's probably going to be a "Wii 2" or other next-gen system from Nintendo at some point, so I was saying that I would want to do another game in the series on the next Nintendo platform.
"I want to continue the No More Heroes series, but I've finished the story for Travis. It's completely finished in No More Heroes 2. So I want to think of a different story -- a different character, or something like that within the series -- starting on the next Nintendo platform."
There's loads more of Suda goodness through the link there.