if your interested here is a YouTube Video of Chad Nelson ( Developer, President) where he touches on what goes into making a course plus allot more:
http://www.youtube.com/watch?v=2JmwxDFZknw
As for new courses, according to Wikipedia :
"In line with its USGA tie-in, WGT has published a preview of the new
courses that will be launched on the site in conjunction with the
upcoming locations for the U.S. Open: Congressional (2011), Olympic (2012), Merion (2013), Pinehurst No.2 (2014), Chambers Bay (2015), Erin Hills (2017) and Pebble Beach (full course) (2019)"
I don't think this means no other additions will be made that are separate from the USGA but at least it gives you an idea.
Hope that helps a little
Unibooboo