How would you market this .....?

First thing is I would hire paid posters to make it "active". Nobody wants to participate in an inactive forum, wont happen no matter how many people you get to it.

Next, I would start promoting links to "active" threads that people may want to comment on, which requires them to register, etc.

If you know some players personally, maybe friends, relatives, or co-workers get them involved by talking to them.

If you can get people from teams which are bitter rivals it would be even better.

The more lively you can get a few threads going, the more people will "stick" when they visit.

Nice question. Here's what I would do. "CLOSE DOWN THE FORUM"

1:
Yep... you knew I was going to say it... Start a football blog, but the only difference is that I would make it a little controversial.

(I don't know anything about the sport, but I do know this; the fans are rabidly passionate about the sport and their teams.)

example Posts:

If player so and so would learn how to kick, team A would have won the the world cup? (Pick a popular player -go into a little detail)

Which team do you think should burn thier uniforms. Don't you think they are the worst team in the league? (Pick a popular team - go into a little detail)

2:
Either go to Ebay and see if you can pick up a little something from any professional team, or look around the internet and see if you can get your hands on something that a soccer fan (oops, football fan) would want (love) It doesn't have to be expensive, just meaningfull. Or create it yourself.

3:
Go to Youtube and see which football videos are highly viewed. Embed some of those videos stratigically placed in your blog.

4:
Using the item that you got off of Ebay or Where ever you fond it as a prize in a contest that will be held on a specific date.

5:
You can find Wordpress plugins that will sign people up to an email list.(Of course you want to let people who are making comments on your blog, know that they will be automatically place on a list, so you can notify them on when the contest will be held.. and when they can come up with thier best arguements or opinions on your various blog post. (from inside of the forum)

If you can't find a free plugin that does the job, or you don't want to spend any money, then just try to generate enough excitement about the contest.

6:
Here's the catch... You will only allow people to enter comments 2 or 3 times to get the ball rolling on some of the controversial posts that you put up.

They can only post more comments and enter the contest, when they register for your forum. And they can only qualify to win when they have posted a certain number of post inside of the forum. And you will pick the best one and award that person the prize. (or you can have a few cool prizes)

7:
After you have setup everything... start driving traffic to the blog.
If you can get enough people making their couple of comments on the blog, and waiting for the time when the forum will open, you will be building antication.

Tell them that the contest will only be open for a short time so everyone has a chance to enter.

*note:
Tell them to tell there friends on Facebook & Twitter.

*note:
While you are doing this you will be putting up more controversial posts - and sending out emails letting people know when the forum will be open. If you do it right, The people who come to the blog will be generating content for you.

*note:
If you did not collect any email addresses, then just set a day when you will open the forum and make sure that everyone knows when that Day is.

If you do this right... even after the contest is over you should have a popular blog & forum.
Remember... Tell them that they will be disqualified from the contest if they post more than the allowed number of comments on the blog. (I'm affraid you'll have to manually approve comments for awhile in the beginning. But this should train them to go to the forum when they want to post. Also, when you think you have enough people inside the forum, just close down the blog and lead any new-comers there.

(Award the prize to the winner, but try to find or create something, anything football related to give all of your participants in the contest.) But, to do this, now you are going to have to collect some emails.... or make a hidden page that they can go and get their gift from.

Well I hope that gives you some ideas.