I don't know how legitimate this is or not, but
thebadwolf91 read in her DW magazine that series five will not be series five, but will be series one. Which makes NO FLIPPING SENSE! Rebooting and starting at series one made sense for the relaunch in 2004, but now? I can't think of any reason for it other than Moffat's colossal ego. *sigh*