I do not have a specific story idea that I do not feel like I am able to pull off; that might be because I generally have an aversion to fantasizing about writing books that are far out of my capabilities to write. But for the ones that do, they usually land in one of three categories:
Requires Lots Of Historical Research
I want to write historical fiction, but that is hard to do. Even with the internet, finding the very nitty-gritty details of historical matters are the hardest part. Not general details, like how a battle went, but things that are far more specific. How did the life of a sailor on the Union's Mississippi River Squadron fare during the Civil War? That kind of information is information I will need to go to a niche historian for, and I know none with that field of study.
Paradoxically, I am also much less confident writing about the near-past. Think the 70s to the 90s. I never lived through those time periods, and if I get anything wrong there definitely are people today who have lived through those years, and could call BS on things I make up. So, other than interviewing people who lived in that era, conducting research is also somewhat difficult.
Requires Lots of Scientific Research
Hard sci-fi is fun. I like thinking of both near-future and far-future hypothetical scenarios. But if I want to be accurate, I'll need to go to sources like Children of a Dead Earth, where you practically need to be a nuclear engineer, rocket scientist, and weapons specialist to understand a game like that. I eventually want to eventually write a story about Paperclip Maximizers, but that plan is far in the future.
Requires Me To Be Very Intelligent
Heavily logic-based fictions, where manipulating people and stepping through philosophy are the norm. I have a friend who has an enviable mind when it comes to logic, and poking holes in logic; figures that he is also in the tech field. I've seen him do terrifying things in games, homebrewing his own exploits and ripping holes through anticheats, and he applies that same line of thinking to stories: "Why can't [the protagonist] [play with the laws of physics] in that setting?" I suspect that he does not enjoy a lot of media because of that, but I typically refer to him if I have a hunch that a storyline of mine has a hole or two. He's good at seeing things like that.
I do not have that kind of brainpower. I wish I did, but a logicfic is completely out of my realm of ability to make.