Do you have any beta sign ups? If there is a recurring theme that might make it easy to select a niche. Another thing you can do is select a niche, any niche, that you are already active in, like gaming or cooking or something like that.

Hacker bloggers is not a bad place to start if you are already active on HN and other places. The only thing I would caution is that hackers tend to be much more critical than the average person. Like some are willing to be early adopters but others expect everything to be perfect. Expect to hear a lot of "You should fix this bug/implement that feature - it should be EASY!" I think that tech-savvy but non-hacker people might be more open to "Is this useful?" discussions.

As a side note I find your logo to be really unclear. Honestly I think you should change it if you want it to really catch on, but if you're not willing to do that, then you might put "jellli.com" text underneath it.