Jemima West of Borgias fame has been cast Isabelle Lightwood.
Personally, I have no problems with this.
But only time can tell!!
p.s. it is rumored that Chris Riggi, who played Jacob Black in the Twilight parody, Vampire Sucks has been cast as Isabelle's brother, Alec Lightwood. If this is true, it is a perfect choice!! The next perfect choice to that of Lily Collins as Clary.
My vote for Magnus Bane?!?!