A ranking system would need several things: A large enough committed playerbase. I admire you for attempting this, but I do fear that the size of the hardcore playerbase will limit it's potential at this point. The other is that I would recommend sorting the guilds by type, namely casual, levelling, endgame PvE, endgame PvP, and twinking. The other issue of course is that some system of moderation would have to be developed, especially for the quality of complaints that any system might get. You're likely to get bitter players.

Finally, for the system to work, a significant percentage of the entire PL playerbase would have to be committed to the rating system and accept the legitimacy of the guild ranking system.