Kinda' like wordpress with plugins/themes. People will be able add/remove and customize plugins and also change their theme (ui design).
Some browser games that are action based MMOs with open world pvp:
Crystal Saga - http://www.youtube.com/watch?v=QbcyKDMMJEQ
Drakensang Online - http://www.youtube.com/watch?v=XaQllGWkp5g
Grand Epic Online - http://www.youtube.com/watch?v=PxkwrQ0ym98
Ps: I have 7+ years in php, mysql, javascript, html/css.