I for one would love to see a fully implemented Seasonal and weather system for Skyrim. Something that can add a great variety to old landscapes. From having seasons where there is little to no snow at certain area's, then seasons when it can come down hard, etc.
Basically depending on what Beth wants for Skyrim to look like they can simply implement a system where each season of time that passes the weather/landscape changes depending on what area it is and what time of year. So say, in the summer there would be lno snowfall in some area's (wehreas further north or up high enough in the mountains) there could still be some. Then come winter time there can be snow fall where it was in the past simply a nice lush green field.
All it would basically take is to program the weather and landscape to reflect the season along with a variable weather system. Where they can have various types of weather (rain, snow, ice, etc) and then varying degree's of it (hard, light, etc) and then plug those into whatever season they want to show them in with a higher % of happening. So say, in the winter time there's a 90% chance throughout to see snow, in the summer time there's more rain and no snow,e tc.
Apart from this, another major thing that would be nice is if there was a chance, a very very small chance, that at some point in time a certain random chance that there could be a environmental change that impacts teh game world in some way, perhaps opening a new previously unexplorable area or blocking some well known passageway and you have to discover a new way to go.
Like for example, you are travelling out on a very dangerous mountainside. It's came a blizzard, not just anormal snowfall but a huuuuuge blizzard that is quickly building up snow. the pass your normally have to take to get to the city you were travelling to is blocked now so instead you have to go through a new area, which previously was closed off, but because of the blizzard an avalanche has opened it.
You get the point, basically very rare weather or other events that bring variety to the game world and makes your travels along it seem more varied and exciting.