Something I'm sure would be a hit would be a "smart" content spinner.
Meaning, unlike the junk out there, something that could re-write blog posts/articles in a much more advanced way.
Basically, achieving "morphing" type results WITHOUT the need for the person to specify words to swap in.
ie. instead of writing stuff like [word1|word2|word3], it automatically finds the nouns, adjectives, verbs, etc... and replaces them by similar meanings, via Thesaurus APIs and such.
It also would allow for reserved words not to replace, specific replacements (ie. if you always want to replace the word 'apple' by 'granny smith'), etc...
That would be a hit IMO
