I would love to have the ability to create day cycles with a rising/setting sun and moon and the ability to transition between skies (dawn to dusk to night). Being able to set a 24 hour day cycle would do a lot to sell an immersive RPG environment and add some much needed to life to the world. Of course, having control to the extent that people who want to create an environment on another planet with a longer or shorter day cycle would also be ideal.
Another component that would add to environments would be the addition of some sort of weather control settings. Like in World of Warcraft where, in the right climates, it might start to rain or snow seemingly at random. Of course, this might come later with the introduction of particles and some more advanced scripting. But some sort of weather system would be amazing.
I'm not sure if either of these subjects have been discussed, I've been trying to make my way through all the old update/meeting notes to find out, but haven't seen anything yet. So, if someone has already asked, apologies for repeating the question. :)