That was a staple for the original Delte's List. So was the need to sleep. I must admit, though, that I completely love the sleep system that the developers have put into vanilla Skyrim. If you're the type of player who does not want to sleep, then you're not penalized (as all of the Sleep mods for Oblivion did). But, if you do want to sleep, you get a neat little buff that makes your skills improve faster. The more rested you are, the better the buff. For example, if you are "Well Rested", then you get a 15% bonus to your XP for 8 hours, until you start getting tired again. This bumps down to 10% when you're only "Rested".
You'll definitely want a slower-leveling mod if you want to stick with that. Between being Well Rested, and the various guardian stones, skills can level quite fast.
I do like the benefit-for-doing over the punish-for-not-doing approach, though, particularly when it comes to werewolf and vampire characters since they could have trouble sticking to a regular survival mechanics (resting, eating, etc). This way, they still get "punished" by not getting the bonuses, but they don't become gimped (they simply trade being able to regularly get certain bonuses for unique buffs and abilities).
I'd like to see something similar with Hunger mod. If you don't want to bother with it, you don't have to. But, if you want to eat and drink, then you get a little benefit of some sort if you're not hungry.
I think the cooked food buffs really needs to be redone from scratch. Make cooked food have much stronger effects than simple instant +10/+15 healing at max. Good recipes should provide boosts that last for hours (whether it's a fortify health/stamina/magicka that slowly decreases over time, or increased regen rates that slow back down). But a downside: keep food too long and it spoils.
Another thing is smithing. You can use grindstones and workbenches to improve your weapons and armor, but wear and tear from use should slowly undo those improvements, encouraging you to go back to the grindstone/workbench after getting more of the required items.
Also, I think crafting armor could be improved so different types of pelts can create different types of armor. Killing a snow wolf for its fur should ultimately allow you to make snow wolf armor, while a bear pelt will lead to bear armor. Currently, no matter what kind of pelt you get, it becomes ordinary leather which leads to the same types of armors.
Also, if possible, I think it should be illegal to use private crafting stations without paying (unless you're friends). Doesn't seem quite right that a complete stranger can walk over to the nearest shop's forge to make whatever.
Also weapon rebalances. There's no reason silver weapons shouldn't be craftable or improvable, especially since there's werewolves, vampires, ghosts, and zombies roaming about. Starter gear should not be so strong (more of a gameplay thing than immersion, but still). "Ancient Nord ..." weapons should not be as good as Steel, being that they're, well... ancient and rusted.
Also, elf and beast race children.
Also more general pvssyr and NPC interaction*. Skyrim cities and towns feel so lifeless with how 90% of the NPCs just brush you off when you try to talk. NPCs are quite willing to divulge their troubles and life stories if you happen to get within five feet of them, but I guess they're just not comfortable with general conversations. You should be able to ask more NPCs about rumors and jobs. More NPCs should be able to provide radiant quests to work up friendships with them (a lot of the dialog is shared among NPCs, so making some of those befriended NPCs marriage-potential would also work).
* I guess some people would argue "it's not realistic that everyone would want to talk to you!" I'd counter that with "it's not realistic for a capital city to be populated by a couple dozen people." It comes down to scale. I imagine it being that the unimportant people aren't represented in the game, as they'd just be useless waste. The NPCs you see are the "important" ones that will interact with you.