Would be neat to have seasons in each zone they cycle about every month of rl time im not sure if Unity can do that or even if its possible??
The idea of seasons is an ambitious one. It requires a lot of drawing and alternative surroundings for the artists/dev's to provide. But yeah it would be very nice and fun to play in.
Swamps, tropical forests, forests, fields with pools of mud, canyons and cliffs, those would be my favourite.
There isn't much I wouldn't like, perhaps oceans of desert dunes would feel somewhat boring, unless you put some ruins or other things in to give it more a sense of distance.
I would like flying mountains only if it's implemented decently; so no teleportals from one isle to another. More hostile and less of an airstrip with npc next to it. Perhaps ropes could connect one isle to another.
But I'm diverting here.
Although spring is a season not a terrain, I do like to travel during the daylight through open grasslands with kneehigh vegetation (and the hidden loot/mob/npc ones every so often). An occasional rockformation and wildlife jumping out when nearby.