It'll be SS, possibly SwSh, just like every other generation in the past. People who tried to push SuMo were in some Berenstain Bears timeline. The only, ONLY, time a letter has been added was when two of the same generation shared a letter (Pearl and Platinum) which resulted in the latter adding the next letter. RBGY, GSC, RSE, DPPl, BW, B2W2, XY, SM, USUM.