The 5DS Mark 2 would be an ideal camera as the more expensive RF mount body. You don't have to worry about fast throughput on most of the subjects you would should with a regular 5ds.
I'd slot in an SL level camera as the 4th one if any. I wouldn't be surprised if no new Full Frame DSLR's came out. If one did, I'd guess it would just be a 5dV as the last mirror slapper in the line. The RF mount 1d would come out 2020, and possibly a 6dII equivalent with faster throughput to be an equivalent sports shooter to the 7dII.
NO, moving the 5DsR II to the gimmick/trend of mirrorless would NOT be ideal. As I said in the other thread, this would be a horrible idea, and would only benefit people with dainty hands and/or weak arms. You lose OVF, battery life, and a properly sized body for balanced use with large L series lenses.
As someone else noted, yes mirrorless can be made at a lower cost. Ultimately, the mirrorless trend/fad is about making cameras cheaper, and to a large extent smaller as a reaction to sales lost to smartphones. The former is a very bad sign for where future cameras may be headed quality wise, and the latter is an exercise in futility.
Canon would be wise to continue both 1D and 5D cameras (all models) with mirrors, and to also continue with at least 2 APS-C cameras with mirrors. The 6D would be the best FF candidate to be discontinued, as it was already a compromise FF for people with smaller wallets. As such the lower pricing that should happen with lower level mirrorless will be appealing to many of them.