Alternate rhyming dictionaries is actually a really interesting problem, as different cultures have different ideas of what the ideal rhyme is.
In English, words where the onset consonant is the same (e.g. 'tend' and 'attend') are considered relatively poor rhymes, whereas in French they're considered to be "rich". In Portugese, even the part of speech is taken into account: a rhyme between a noun and a verb is considered a better rhyme, say, than two nouns that rhyme.
This is obviously a solved problems (I mean, you can buy rhyming dictionaries in France or Portugal), but I wonder if it would have any interesting ramifications on the design of the feature.