As far as contect.... I always did my publications on my own terms. If I didn't like an album or an artist, I wouldn't put them in the publication. But I was doing a regional thing, whereas you're doing a nationwide thing so to get a fanbase you'll have to include things you don't really like.
You can try to include features that other magazines don't, but the tried and true magazine format has been working forever and will most likely continue so you can't really go wrong with a good mix of interviews, reviews, profiles, articles, industry news, etc... Also see if you can get top 10 independent sales charts from the more popular record stores in the key markets your magazine hits. That way people can see what the underground scene is picking up on around the country.
Equally as important as the content though is the presentation of the publication.
First off, make sure you have people on the team who can actually write. Second, try and go in-depth with interviews. Go past the obvious questions (Who do want to work with?, etc...) The obvious questions are fine because people need the general information, but follow those questions up and see where else the interview might take you. Third, (and possibly most important) make sure someone with a good eye edits every issue for spelling and other errors. There's nothing worse than a publication full of errors.
Hope I helped.