So with Sina and Dexio being in Alola and sporting Mega Rings, the Jungmo-o description in v-jump mentioning Mega Gardevoir and Mega Glailie appearing in Sun/Moon merch for 2017 we can safely say megas are back

Now all those are very recent so id say its a safe bet that the next trailer on 4th October will be megas reveal.

GF has covered most stuff we need to know about Sun and Moon. The remaining stuff of interest they might be revealing before the games come out are Megas, Starter evolutions (at least middle ones), more Pokemon, Alola forms and Ultra Beasts (but those can be squeezed in any trailer) and post game activity (if there is any good one)

The biggest ones of those are starters and megas and i thonk we'll get megas first. Possibly new ones aswell. Afterall the whole campaign for XY had no megas untill 2 months before reveal (except Mewtwo which was just "form" for a few months) so having a big chunk of new megas in October and November isnt anything odd

Corocoro promised news on Type null so we wont see that in the 4th October trailer