First you tell us what your site is, what functionality you want etc. No one can give you an answer without some info. Thanks.
Well I'm actually doing a php based game too (in my free time which I never have so it hasn't gotten anywhere...)
My advice is to just start adding features one by one onto the member system. I don't know how your site will work but if you had things like training, fighting, and chat you could just start with one of them and try to get it to work then move on to the next. It's all dependent on personal preference and individual needs though.