Dwarves. Given the sheer industry, weirdness,  introversion, and potential power of this race I've never felt like I've done  them justice. Though I'm making an attempt right now. Through the use of  underground cities with unique architecture that the Dwarves designed because  they were forced to, not because they wanted to.
  
 Sea-going adventures/voyages/explorations - I've  designed some good ones, but I've never done, for instance, an island hopping or  coastal exploration campaign like I'd like to do.
  
 Lawful Evil. To me there's no such thing. But I'd  like to be able to create a Devil who is to me as interesting, as dangerous, as  vicious and malicious, and as outright scary as a Demon. But the lawful part  makes them seem like Gentlemen pansy-demons. It's like they're all Victorian era  ring-boxers instead of vicious street fighters who will pull a shank and kidney  spike ya with it. I don't really play alignment, and don't want to, but I like  the concept of Lawful evil in theory, especially as a motivation for a monster  like a Devil. But the lawful part of evil strikes me as impossible. It's like a  governor you put on Evil.
  
 The Giants - I've designed and played some good  Giant adventures. But I don't think I've ever really used their true potential.  They have a lot of potential. In my setting there is a half-giant race the  players can use a character. That's has gone very well. But giants as a culture  and a race, I just haven't nailed that. A friend of mine did - he made giants an  alien race that had crash landed on an island in his world and couldn't escape  and so had slowly converted their technology into a culture different from the  rest of his world - but I didn't wnana just copy his approach.
  
 Mind Flayers - The friend I mentioned above had a  big campaign I played in involving the Mind Flayers. In his campaign Mind  Flayers (this was back in the late 70s) were a group that had invaded his world  from a different dimension, had a very bizarre mental based technology, had run  into the giants and a war had broken out between the Mind Flayers and the  Giants. The Giants used what we'd call science, the Mind Flayers used mental  technology, and we (the players) had magic, with the local native populations  caught in the cross-fire. His Mind Flayers were truly evil and impressive and  extremely dangerous to everyone, the Giants included. From that day forward I  wanted to do something really impressive with Mind Flayers but they never seemed  to mesh well with my setting. And again I didn't just wanna copy  him.
  
 Psionics - I've altered psionics a good deal and  now include it as the abilities of certain creatures. But I've yet to be  satisfied with Psionics as a game factor. Or a system. It always seems it's the  fall back position, and is never used to potential, and I've never found or  designed a system I think meshes perfectly with my setting.