I do not think blog networks are necessarily bad but they won't make your blog the most popular on the web either.
Definitely do the subdirectory as you mentioned, and use WordPress. There are other platforms out there that are just as good but I personally prefer WordPress and you can't go wrong with them.
Continue the engagement with other soccer blogs you mentioned and start asking for guest posts with links back to your blog.
But, above all, write the best content there is to write about soccer! I just so happen to be a huge soccer fan living in the U.S. and I can tell you that there is definitely a lack of quality content on the web for U.S. soccer fans.
I am assuming that you target the U.S. market, and if so, then you have an awesome opportunity to make your blog the best thing out there for the U.S. fan.
Good content isn't enough to move the SEO needle for your site. Premium content is a must. It's a lot of work, but if you do it right then you'll see huge benefits. Don't think you can just plop a few posts down a week and see a difference. You need to become the central source for all things concerning soccer, and that will require a concerted effort by an entire team of people.