As an owner of a 1DX Mark I and Mark II I think we need to know a little more.
1) Glad that it takes the same batteries, but as I have quite a few I would like to know what impact using Mark I and Mark II batteries in the camera will have and whether the new charger will handle all three versions the same way? The batteries are amazing and if they have improved them further, then wow.
2) Love the illuminated buttons. This will be very useful in some circumstances, and I hope that it can be turned off, and the brightness dimmed etc. too.
3) Lots of fancy words about the AF flying about. It's not going to be worse than previous versions, so will be great to learn more about this.
4) They say it has WiFi and Bluetooth but there are pictures of it with the WiFi adapter, so am looking forward to learning exactly what this means. Would be truly fantastic if I can send RAW pictures directly to the cloud and / or my phone without too much battery drain.
5) New cards. Ouch, if you shoot video - especially 4K - you are going to spend quite a bit of money to get cards big enough to fit your work. Add in a second or third for redundancy and that's a fair bit of change. I liked the dual system on the 1DX Mark II as it meant I could still use my old CF cards when the primary card was full without spending more money. Plus, there's no way of knowing whether these CF Express cards will be in the 1DX Mark IV. Personally I would have liked to have a CF card slot as well as a CF Express slot.
6) The body has hardly changed at all so that's good.
7) FPS - I have never locked my mirror in place to take advantage of the Mark I or Mark II's fastest FPS when using viewfinder. Will be good to learn more about the FPS and the viewfinder too.
I can't help but wonder why Canon has made this development announcement? From memory, don't they normally just release ?