I have to be truly honest. I just got a 1DX about 6 months ago to replace my old beaten down 5dMK3 and I cannot honestly think of anything else I could want from a camera. It has great AF, great low light capabilities, excellent ISO quality. What else is there that you "HAVE" to have? To me nothing. I understand we will always want more in life but for me I have to say I am truly happy with my 1DX. Just my opinion of course, I know most feel differently.
Me too, recently joined the 1d line. Let me put it this way. If Canon releases 5D 4 and 1dX II tomorrow, I will more likely upgrade the 1d over 5D. I will keep my current 5D III as a second body until it dies or there is a huge discount on 5D 4.
The grip on 1dx is a HUGE advantage when shoot with larger lenses(85L, 70-200 f2.8 IS II, 400mm etc...)
After shooting with the X for 2weeks, I now feel I need to add a grip to my 5D III. It feels naked at bottom without grip.