I'm fond of a Evolutions-On format because it removes Professor Sycamore, N, Volcanion, Volcanion-EX, Dual Type Pokemon and things like Max Elixir from the format, cards that should not exist in Sun and Moon. I'm a little less adamant of this because they are using a ban list and can just place these on the ban list but I find these cards a threat to a Sun and Moon format.
That said, no one actually knows what the format will be and the only safe thing to do is buy cards from Sun and Moon on since the format could be Sun and Moon on. People are overlooking one fact here. There is a reason they haven't announced what it is right now and I feel it's because it will be either Evolutions or Sun and Moon on and they don't want to say that as of now because it would run sales of each BREAK set, Fates and Steam, which is five more sets still on the market that will lose any value. They want to sell these as well as their Sun and Moon sets.
Breakthrough-On is likely but evidence suggest it's something else, Evolutions-On could be it since rotation is often at the reprint set, which Evolutions is (as well as there being no broken or overpowered cards in the set that complement Sun and Moon sets) but, the safe best is Sun and Moon, since most Trainers worth having were reprinted. The lack of a discard, draw seven effect and nerfed versions of N and Lysandre are also telling... I guess Team Flare Grunt too...