Well it has changed from code convenience of 30 days per month to what is in the gregorian calendar. For for Skyrim, the only next valid change would be to include actual leap years, which wasn't feasible if we're able to reach lvl 40 or so in a hundred days or so I think there are better alternatives to letting time pass by than changing the lore. And well, I hope holidays and celebrations return somehow.

I'd also like to see terrain affect movement speed. Walking or running in snow should drain fatigue faster than walking on a flat dirt road. Likewise, trying to move through snow, mud, thick undergrowth should reduce movement speed.

I guess I'd like to see a BALANCE system, where, depending on Agility, your character might fall down. Clumsy characters might have a small % chance of falling flat on their face if running on even terrain. Hilly, slippery, jagged terrain would increase the % chance to stumble. Higher agility reduces the % chance. In other words: no more running full speed on a 30 degree slope of snow and scree.

As long as it allows us to have seasons, then sure. But as I said before, I'd 1000x times prefer to have 15-20 days instead of 30, and then be able to have seasons; then to have 30 days and have no seasons at all. Holidays and celebrations would be cool indeed, if implemented correctly. It would make things a bit more interesting and varied.
Still, real seasons would be groundbreaking for Skyrim, groundbreaking for any new game

It may also be a new application of the athletic skill. Or, another option if survival-exploration become more important, a brand new skill like navigation or survival.
It would be great if ranger or druid characters could really have an interest in TES at last.

Details like leaf change colour in autumn and eventualy fall off and ride away on the wind would have been beautiful. Don't think details like this will make it into the game, but I like the idea.

that would be really really cool the leaves on the trees could change crops and NPC scheduals would change accordingly