If you mean with forum a site like TeX.SX (which is not a forum but a Q&A site) then you should nowknow the following:
All stackexchange sites need to be officially proposed on http://area51.stackexchange.com/ where they need a certain number of supporters to get the site into the beta stage. Once the beta site shows enough activity it is elevated to a full site or shutdownshut down otherwise. It isn't very easy to get so far if you don't have the right amount of supporters and your proposal can be closed as a duplicate similar to questions here. Because TikZ/PGF is for LaTeX and there is already a site for that, I can basically guarantee that such a proposal will be closed quickly.
There is already a "PGF/TikZ forum" in form of a mailing list which is accessible under http://old.nabble.com/PGF-and-TikZ-f3582.html. Also http://www.texample.net/tikz/resources/ is a dedicated TikZ website.