I've been thinking occasionally about a letters challenge.
Right now, I'm thinking about 4 different types. (Extreme) Regular Letters, Single Letters, Reverse Letters, or (Extreme) Words.
Basically, with this you'd get a random set of 6 letters. You'd then pick out your pokemon based on them. You'd pick a pokemon who's name starts with that letter. Single letters is you pick one letter and only pick from that Pokemon with that starting letter. Reverse is... Well, I'm not so sure about that one. Would be to hard, since in this case you'd only be able to pick from ones who name ENDS with a picked letter. Words is a play on Letters. Basically, you pick letters that form a word.
For Words you'd be limited to however many letters is in the word. If its 4 letters, then 4 pokemon. 5, 5 pokemon. HM slaves wouldn't be allowed for any type of challenge though.
Extreme would be slightly different. Basically, you pick the first Pokemon that starts with a selected letter.