I don't think there is currently any store like what you've described around here, which I've always thought was a shame given how we are in a major metropolitan area. As you've mentioned, there is OMOI, and the Fye in Center City has a small selection of manga, cosplay accessories, posters, and T-shirts. When I'm not at cons, I tend to buy most of my anime at Best Buy (I'm in their rewards program), and most of my manga at Barnes & Noble. I generally wait until cons to do shopping for other types of stuff, because it isn't very available otherwise.
This is all to say, I think it would be great if you opened a store in the area to cater to the needs of otaku
I think, if you can afford it, Center City would be a prime location because of its accessibility and the fact you'll get a lot of walk-by traffic.
As for types of stuff I usually buy, it is mostly anime, manga, OSTs from anime (or CDs for Japanese artists I like), wall scrolls and posters, and T-shirts and messenger bags. I could see figures and knick-knacks being very successful also, as well as perhaps Japanese games that aren't available in the US. I don't personally buy a lot of cosplay accessories, but I know there are folks who do, so it would be handy to have a local supplier.
One other random suggestion, if you have the space, is to perhaps have a "chill" area where friends, meet up groups, clubs, etc. could come to hang out. I know there are a variety of otaku-related groups in the area, and if they utilized the space, you would have a regular influx of folks who are already interested in your wares and may be tempted to buy