Flow Festival making a first announcement tomorrow so could be some crossover (although they will probably include some artists already confirmed for WOW).
Post Malone is focusing on his new stuff during his upcoming stadium tour here in the US, but I really doubt he would do the same setlist on a European festival tour. He'll probably go back to a hip-hop focused set for the summer, he might play "I Had Some Help" just to have something new to play, but I don't see him playing too much "country" music at a festival.
Agree, his Coachella set will likely set the tone, but even then he will use Coachella to springboard into his own headlining tour, so it might have more new stuff than he would play in Europe.