You know, I think Daniel Day-Lewis could do it, and with very little makeup too (except for the parts when Emerson is an old man). I don't want to sound like I'm typecasting because of "Lincoln", but I really think Daniel Day-Lewis and Ralph Waldo Emerson have a similar shape to the face.
As for whether or not a movie about Emerson's life would be interesting, it would all depend on the writing. It could be fabulous, but it could also be absolutely boring. For example, I really love Lon Chaney. I also love James Cagney and was really looking forward to watching the 1957 biopic "Man of a Thousand Faces" where Cagney plays Lon Chaney. Cagney gave a beautiful performance and they did a good job recreating the monster makeup; but on the whole the movie was an over-acted melodramatic mess. It kind of left me with that "Why did I watch this?" feeling at the end.