VGC18 is going to allow the entire National Pokedex, as long as the Pokemon were obtained in Alola. All items are also allowed so yes, you can use megas. What this will mean though, is Amoonguss is probably going to run all over the place because it checks 3 of the Tapus, and Mega Salamence will be the go-to mega due to how broken it is. It may face difficulty because the Tapus are everywhere, but still. Aegislash will also make his triumphant return as well as a powerful special attacker.
The direction the meta will unfold though depends on whether they're going to dump all the legendaries on us like in ORAS. If so, then it'll be pretty much Battle Spot Doubles, but if not, this is going to be one diverse meta without most of the classical legendaries.
