Most RPG's don't do seasons. Rain, snow, are definitely in the game. Just need to be in the right zone. It's not gonna rain in the Badlands. Snows all the time in Winterspring and Northrend. Night time can be really annoying in the new areas where it can get too dark (but not as obnoxiously dark as The Division 2, even with brightness turned up you can't see shiiiiit at night).
They wouldn't have to completely change art assets to make seasons work. Just a transition between current texture and pull that seasonal texture from the other areas. Might not mesh up perfectly, and implementing it in the current wow code would probably suck balls for developers, but they've been enhancing the engine on nearly every expac so who knows... Water, mists, lightning, fire, smoke, fog, poison... Compare that shit to the textured water surface of vanilla wow and short animated flat texture fires, no lightning, no mist, no smoke, no fog. And trees. Holy shit has the quality of trees ramped up, too. It's so glaring when you fly through darkshore or unguro compared to zandalar.
I really don't understand why they don't refresh old content and out new quests in it. There's so much of the world everyone hasn't spent time in unless they're veterans, and even still, I wouldn't mind tracking assholes down old areas. Like they're doing with Darkshore and Hinterlands, but on a much more expansive scale, as in, fill some old zone with as much content as they out into Broken Shore...as long as it doesn't replace new areas entirely, but if you made an old area look fresh and new, I wouldn't give a fuck if I was doing quests in an area I'd been 100 times before if it looked completely refreshed or changed based on the current world environment/story.