I think people are overrating Marshadow. There will be plenty of ability lock to deal with it, Alolan Muk, Garbotoxin, Silent Lab (for wcs at least), ect on top of it being low HP and psychic weak. I'm not seeing much of a reason to play it outside of expanded where it will be quite good in NM as people repeatedly mention and will probably make that the dominant deck since it now hits dark SE instead of being resisted by dark.
Po town suffers the same issue all stadiums that do something on your opponent's turn suffer in a format with field blower, your opponent will typically have 4-6 cards in his deck that can get rid of your stadium before it actually does anything for you which makes it a wasted play.
In terms of what I think will be good, I think people are really underrating Ho-oh-GX by focusing on trying to figure out how to use its GX attack. Why do that when you're hitting up to 210 with band or one steam up on turn 2? Everything with enough HP to survive that is either much slower than Ho-oh/Kiawe or is fire weak anyways. As far as GX attacks go, Turt's GX attack is basically perfect for recovering from a downed Ho-oh. Guzma also works quite well in Ho-oh because it lets you get the Lysandre effect, then switches you out to reset your effects. Switch to something with a float stone, bring it back in and attack again.
People are also sleeping on Necrozma and Weavile, the game is getting more and more spread damage and the baby Lele they revealed awhile back is perfect for that kind of deck. Necrozma will be key at worlds since it hard counters megaray in particular.