Heather doesn't have to be fought on Mountain field. What if Reborn will put Sky Field from Rejuvenation for her fight? It is a possibility. Mega Salamence is a 100% for me, for the rest I imagine that she can have a Crobat, maybe even Corey's, rest is not clear yet.
For Bennett and Laura, her team will be Chlorophyll oriented, so stuff like Venusaur, Leavanny, Lilligant (her ace)
Bennett was butterfly themed, but I believe he could put some different bugs, and I can see Pinsir as his Mega and Sceptile for Laura.
Elias, I believe he will have a Ditto, as for rest... not so sure. There are some good hitting and bulky mons like Snorlax or Slaking, however his choice in Megas is limited to Audino, Pidgeot and Kanghaskhan (if I missed a Normal mega I am sorry)
And lastly Anna, Starlight Field is like a double-edged sword, it boosts Psychic mons, but also Dark mons, her weakness. From her team I can see stuff like Starmie, Girafarig, Nostra of course (her Jirachi), but as for Mega, I believe Alakazam is an option, but I can also see a Metagross, since its signature move (Meteor Mash) is boosted in this field.
These are my opinions on E4.