It's hard to say if SM will add anything to warrant a "new timeline." As it stands, the whole "multiverse" thing presented by Zinnia was mainly just a writing cop-out to lengthen the plot of the Delta Episode, so I'm really not too concerned. If the narrative of SM are even remotely well-thought-out, their relative timeframe and whatever universe they're in will be clear enough.
The only thing I can think of at the time being is SM will take place at the same time as XY, or a little bit after, depending on what sort of role Zygarde will play, if it does have its own sub-plot (or place in the main plot).
I will never be able to wrap my head around this, ORAS taking place before XY. If ORAS and XY take place in close proximity (such as the case with HGSS and DPP) then the references to the Kalosian War, Floette's flower in Sootopolis, and even Sycamore's claim all make sense without the need to retcon anything--if Hoenn and Kalos were discovering the use of Mega Evolution around the same time, it makes sense for Sycamore to think that it's exclusive to Kalos because they're discovering it first, with Hoenn right on their tail.