Grey Wardens should not be involved in the Circle. When the blights are finished, so are the grey wardens. Also, they're job is the darkspawn, not policing or managing magic.
What I support is the three group system. Enchanters, Templars, Seekers. Enchanters are mages who decide to involve themselves more in the governing of the Circles. Templars enforce the rules of the Circles while protecting mages and hunting criminal mages. Seekers are recruited from Templars and Enchanters, they are to be nominated by their peer group (Templars nominate Templar, enchanters nominate enchanter) and approved by the senior members of the other group (senior Templars approve mage nominee, senior enchanters approve Templar nominee). Seekers lose all ranks/titles/powers/whatever from previous post, no longer considered Templar or Enchanter, and they oversee the entire Circle operation. So... Enchanters are sort of legislature branch, Templars executive and Seekers judicial... sort of...
So, mundanes can join the Circle, but instead of training as a mage, get trained as a Templar. Circle identity no longer mage only; more like the Grey Wardens, a group with a specific purpose that can recruit from many sources, but the focus is the managing and policing of magic in Thedas. Allowing mages to hunt rogue mages and mundanes still being a large part in the Circle system.