why would they change the name at this stage?
The 5D range has become synonymous with ground breaking camera specs, a marketing person would tell you it would be madness to change the naming policy of a brand. The X for the 1 series solved loads of issues, it's not an "s" variant and it's not a plain mk"*" variant.
I believe the 5D series will become Canon's new megapixel beast instead of the 1Ds range. Why not? It not only killed the 1Ds3, but brought Canon bucket loads of dosh and reputation that money really couldn't buy.
Keeping it to a 5D3 will keep the brand name intact, and delivering around 30mp with a working 25k working ISO, improved AF (say 9 full cross hairs) - and probably dual CF slots to over come the video regs in Europe for around the Â£2200 would bring along another huge raft of upgrade and fresh sales.
If you make it a 5Dx, people will ask what have they done to the 5D range. If you make it 5D3, they know what they're getting - and people will be saying "go on then - what have they done now?" in a manner that will leave you wondering how other brands are ever gonna match it!
That's my 5D3's worth