There are over 100 species in the Federation (150 worlds?) who are some of those other species?, we always read and see the same ones. What's it like for very distant Federation worlds? What happens when a world joins the Federation? What's it like in the Federation council chambers? I would also enjoy a story about an outsiders view of the Federation. Any story relating to any of these I would appreciate. I've exhausted all the Trek lit that interest me and now I need my Trek fix, so I'm turning to fanfic ... but it's very hard to fins stories that appear interesting.
Forget the "ideas." The ideas are already out there. They've been thought up, already. What we REALLY need is proper expression - what's needed are recommendations for these hacks to write better. A whole lot better. To learn how to actually format a story, to learn about what it means to be a writer. Shakespeare didn't have a Word Program. When we got Word Programs, we didn't get Shakespeares. The two have to balance eachother out.
If you've genuinely exhausted all the 'official' stuff (which I doubt cos lets face it there's a shedload of it) and are turning to fanfic to satisfy you then prepare yourself for a phenomenal step-down in quality. To put it bluntly 99.9% of fanfic is complete and utter shite, and wading through it to find something good is a struggle. There are rare exceptions. The stuff by Gibraltar is often as good as anything you'll buy off the shelf. Bry Sinclair is likewise a talented writer of fanfic. There are lots of threads on here by the United Trek Mafia that will lead you to stories of better quality than the usual junk. Ceejay and Darkush fall into that category. Angry Fanboy is a good one to read when he/she/it writes something. I won't list the bad ones for fear of banning plus it would take too long, but steer clear of threads with a hundred posts by the same poster but no one else. If it's good people comment on it. Also how do you get a username like "star trek" this late in the game!?
Alpha Flyer has a series of wonderful Tom Paris stories posted on fanfiction.net. Personally I consider them superior to most of the Voyager fanfic I've read, and many of the pro novels. I found a terrific story that explores Tasha Yar's experiences at Starfleet Academy. Sadly the author hasn't finished that story and it looks like it may never be completed. But what is there is really good. There are some entertaining soap opera-type stories (and series of stories) on almost any romantic pairing you can think of. Orion Press has a series of stories about a planet called Serenidad and its struggles over joining the Federation. This series also explores the reasons why TOS TV Klingons don't physically resemble movie and TNG-era Klingons. It's dark and grim in places, but I found it rather compelling since McCoy has a prominent role in some of the stories, and we also meet an adult Peter Kirk (Kirk's nephew).
You said you exhaust the "lit" by which I think you're referring to the published stuff. You might have missed these. Two of the early Star Trek novels are Price of the Phoenix and Fate of the Phoenix, as part of the main character's back story his planet was invited to join the Federation, there was supposed to be a planet wide debate, followed by a vote as to whether (or not) to join. The end result was the entire planet descended into civil war as the populace divided into factions. Be warned, the earlier novels aren't like the newer ones and there is a sadomasochistic theme running through the two novels I mentioned.