Firstly, there is The Secret Service, which Vaughn co-wrote with Millar and Watchmen co-author Dave Gibbons. Nothing else is known about the graphic novel, other than the handy element that Vaughn holds the film rights. Second on the menu is Superior, which Millar is also writing, this time with artist Leinil Francis Yu. Superior is about a boy with multiple sclerosis who gets the opportunity to become a superhero, with “sources” saying that Vaughn would supervise the script for any film based on the graphic novel, whereas The Secret Service might be more likely as a scripting joint venture between Vaughn and his regular writing cohort Jane Goldman.
So, Vaughn and Millar staying firmly in graphic novel and superhero territory for the time being. In all honesty it is commendable for Vaughn to take his time in deciding on his next venture. Given the relative success of X-Men, he has presumably not been short of offers.