There are various ways to gain members but, generally speaking, mods are not the answer. And if your forum does not a have lot of content then there's nothing to keep one's interest or keep them coming back. That's like baking a bad cake and disguising it with sweet frosting and pretty colors. Yes, a mod like the arcade will probably keep people there but most will only be there to play games and not to contribute to the forum.
Many people think that you can just start a forum and not do much to it and the users will provide the content, postings, etc. Nothing can be further from the truth! "If you build it, they will come" does not work with websites. Just think, when you go to a website you want accurate information, updates to that information as well as any new information, etc. This is what you have to do to your forum and it is a never ending task.
First, you must have INTERESTING content and a LOTS of it. Things that will engage your users and, most importantly, KEEP THEM COMING BACK.
2. Always keep your content updated and regularly add new content. For instance, since your forum is about Sonic and Mario where is the information about each character? When were they started? Who created them? How many games are they in and where is the detailed info about those games? I'd figure that a "fan" site would, at the very least, have that type if info as fans of anything would already know these things or would want to know them.
3. Submit your forum to the search engines. To easily do this, most hosting companies have a "Submit to Google", etc. link in cPanel along with other such things. For a little more info in this please see the following board and topics or just google it or watch the many youtube vids on this subject:SEO/Advertising Info
I highly suggest that you add as much content as possible BEFORE you submit to the search engines. More content and regularly updated content will help increase your rankings with search engines. Search engiens not only crawl your site to make sure it is there but once they find it they keep searching it for new or updated content.
4. Visit other forums. Why should people want to go to your forum if you do not visit their forum as well? And when you do, add your forum to their "showcase" board, if they have one. Also, make sure you display your forum's sig in your profile. Sooner or later someone will click it or ask about it.
5. Have patience! It will take awhile for things to gain momentum. There will also be quiet times...people do have a life, ya know. LOL Use the quiet times to get busy and add or upgrade your content. Cause once it does get busy you'll find that you may not have the time to do it then.
6. Keep it light, fun and not to restrictive. Since you have Simple Portal, create page that have your rules...I prefer to call them "guidelines", it just sounds nicer and there will be time when you might have to bend the rules.
7. Staff rules and everything else regard staff issues should be posted in a "staff only" board. There should also be board for support staff, those who are staff but not admins, and an admin board that is for admins only. I mentioned this because of this topic:http://marioandsonicfanforum.com/index.php/topic,62.0.html
8. Of course, if you are going to have an arcade you'll probably want to add more games...especially Nintendo types.