Well, what are your interests? And, how much research do you want to have to do? I've done baseballones that allowed me to really delve into seasons, trades, etc.; but, that's because I love it. I may soon start a TL that began out of personal interest, but may end up with some interesting changes, if things go the right way.
Next, how much change do you want? If I do the TL I'm thinking, it'll be because I can have some interesting changes. But, change shouldn't be made jsut for the sake of change, either; it's all in how you want to do the TL.
Also, what is your writing style? Some TLs lend themselves better to lots of description, some to lots of discussions, and so on.
If you're looking for something that hasn't been done before, look on Wikipedia and see if anything looks interesting. See how plausibly it could have happened differently. Or, is there some POD that could have been small but then caused something a lot bigger to happen?
Asian PODs don't come along very often. that might be a start. Some discussion came recently about the Russians not colonizing Siberia; I don't think anyone's ever done that. Might that be a thought? If you're Korean, and you know enough about it, maybe you can teach us some stuff about Middle Ages Korea. As long as you make it really interesting, people wil read.