My bet for a potential S6 would be a refined version of the SSSWHRBaro. New gps chipset and other minor under the hood upgrades as with the S9.
The differences between the Ultra and the Sport-lines are essentially the battery capacity and the materials used. Can’t see a reason why they wouldn’t go with the same strategy again, but this time making the only model available the “HR + Baro”, as these features are the standard amongst the competing brands and models.
They might also opt for more clear separation between their top end and mid range products by not including software features such as the intelligent battery and FusedTrack on the S6. These are mainly related to battery consumption and wouldn’t be paramount for the “mid-range consumer”.