If you’re site is going to get its traffic from email marketing I would suggest you to not target on a specific niche.. this will give flexibility and variety in your emails.
However, if your site is going to bring traffic from SEO, I would advise you to focus on a specific niche.. it would be much easier to beat competitors on long tail keywords and to create high value content when you’re focus on a specific subject.
You can try to achieve both ways at the same time (SEO and email marketing), but I wouldn’t recommend that.. will be too hard ^^
Good luck