In the beginning I wondered the same. Someone told me years ago that at first do the niche you like the best. At least while you are learning you are looking at stuff you like
If its babe or teen, for instance, you have a few categories to choose from at most LL's. There would be hair colors, breast size and clothing. This way you are still concentrating yet not submitting to the same cat every day. In there you may find something that clicks. Do not worry if your sponsor is not a dedicated lingerie site for instance. If many of the sponsor models wear lingerie before they get nekkid and so on, it will still be a fit and can still be sold. You can also drop a lingerie sponsor on that site as a cross sell. Somewhere within it all you will find something that sticks, at least for the moment.