No way are they gonna throw out a feature they spent the whole of last gen promoting through in-game importance, merchandise, and even OVAs. They're too popular. The practically created another timeline within the series to accommodate them. They'll be in Sun and Moon for sure. The reason we haven't seen any yet is that it's yesterday's feature. We've already spent a lot of time with the mechanic, and they want to promote the new, eye catching stuff first.
I bet we'll be seeing them crop up closer to release, and I bet we'll be seeing some new ones too. The pizazz of them has slightly worn off due to being present since X and Y, and won't take spot light away from the new features. They'll be treated and received in the same way as normal evolutions. For example, if Mega Bellosom was announced, I still reckon people would be more excited for an Alola variant.
My main concern is how they'll interact with the variants. I'd rather not have all variants share one mega. It takes away the uniqueness of that variant to have it mega evolve into something that's basically a powered up form of its original concept. I don't think anyone would be impressed if the rumored Fighting Alakazam mega evolves into its Psychic form. Then again, it'll be hard work for GF to implement a different mega for each variant. My hope is that the variant can't mega evolve until they've constructed a specifically designed mega for it, such as the Alolan Alakazam being locked out of the present Alakazamite. This'll give time for the the Alolan forms to grow on us and get the most out of them unti GF pull the mega card and make them all sparkly fresh again. It prevents them from being cheapened by taking on the original's mega, and, again, gives us time to get the most out of them.