Seems they're already planning more complexity into the next vehicle than they need to...
I still think they'd be better off going dirt-simple, for now, with surface-to-orbit systems for shuttling people and materials to and from the ISS, while more slowly working on a solid plan to build a permanent base (however small, and not necessarily manned full-time) on the Moon. Or maybe leave the Earth-orbit stuff to the Russians (and soon the Chinese?). Or just buy tickets for their crews from Virgin...

Everything NASA needs to do to get regular trips to Mars going , and establish a base there, can be figured out by starting this way. It'd make more sense to have a big ship, capable of carrying landers, etc. (and never being intended to return to Earth's surface), operating between Earth orbit and the Moon, and later have one operating between lunar orbit and Mars. No doubt the current shuttle design could easily be tweaked to allow it to make lunar voyages, but I don't see the point... this will lead to compromises that will diminish such a craft's usefullness for both modes of operation. It will also be very expensive: I think one of NASA's top priorities should be figuring out how to take some of the controversy out of budgeting manned missions.
New, simple orbit-only ships should support the ISS, and ISS should be used to support lunar and maybe Martian missions. A lot of work needs to be done on ISS to get to the point where Moon and Mars ships can be assembled and launched from Earth orbit, but it'd be a better way to go, in the long run.
Other than providing inspiration, the Apollo crews weren't really necessary, when you think about it ... people shouldn't be going to the Moon and Mars just to plant flags, take pictures,collect samples and deploy experimental packages. NASA really shines (technically
and fiscally) when it comes to doing all of that robotically- why develop a huge expensive manned program
unless a real human foothold will be established? If people aren't going to live and work on other worlds, the risk and expense isn't justified, IMHO, just for the satisfaction of setting foot there. Besides, there will never be another moment quite like the first bootprint being made on another world... as much as the human experience of the Apollo crews changed humanity's collective outlook, that impact will likely never be felt again. It's time to get some real useful work done in space!
It just seems more sensible to me to try to integrate all of the mission-specific hardware and techniques into an overall plan of expanding humanity's reach- moving outward without falling back.