While i would love Day/Night and weather cycles id like to start with a modest request of only weather cycles since they dont really need to do to much extra scripting, only check if is raining/lightly/downpour whatever..
And it would increase immersion and just the cool factor by a lot.

Id love for day/Night cycle as well but if that involves scripting each NPC in the game and extra quests and such maybe ay least add it without extra scripting just for the cool factor and specific quests like vampire related stuff