"need for new, larger maps to fully take advantage of the vehicle transportation"
Artists are badly needed for ALL open source fps projects.
That means that developers should equally focus on improving editors and making import/export between various tools a priority for helping artists; most very good artists aren't programming experts.
Xonotic's webpage is terrible. In order to find out how to submit maps, it is a nightmare. If you want to submit code, it is easier...
Their tool of choice appears to be gtkradiant, but that seems terminated since years.
Googling for gtkradiant shows that darkradiant may be a better fork/choice.
It is not obvious at all for a newcomer. At least not from Xonotics homepage.
EDIT 01: Now I found out that they actually use netradiant - http://dev.xonotic.org/projects/xono.../Creating_Maps
But gtkradiant is the one mentioned here http://forums.xonotic.org/forumdisplay.php?fid=13