4 years sounds about right. Given that the AA filter is a relatively small difference in overall camera build, I'd guess that both versions will be kept. It will remain a niche model. I would not expect a significant increase in MP count, say 55-60. With other cameras the MP increases have been very gradual: 16, 18, 20, 21.
Price is irrelevant. People who want/need a 50MP camera will buy it, the rest won't. Same for high frame rate bodies (1-series), if you need it, you will buy it, if not, there's no point. I pre-ordered my 5DsR, see no need for a 1-series, neither for a 5D4. My first dSLR was the 5D2, skipped the 5D3, got 5DsR, so 4-10 year cycle works fine for me.
GPS was a serious omission. Dual/updated processors will be logical. CFast may be nice, but not necessary. The capture-to-LCD-display lag is rather long, which most likely is a processor limitation, but I haven't yet run into a frame buffer limit that would require a faster write-speed to the card.
The 5Ds/R already has too many features. Most are software, so no incentive to slim it down. It's like Affinity Photo/PS, most only use <5% of features.
I hope that an easier user-interchangeable focusing screen will come back. Given the need for good glass, which Zeiss provides with MF lenses, the need for a matt FS is too obvious. I changed mine, and it is not too difficult, but the ease as on the 5D2 would be better. However, I don't get my hopes up too high, and I can deal with it on my own.