I would say that the new skill/perk system adds an incredible amount of replayability to skyrim (at least for me), and because there are no longer set classes the player is allowed full freedom to craft whatever kind of class they want. So my question to you is: What key changes exactly would make Skyrim more replayable? And I do expect specifics.
1) Gear and itemization model that adds randomness to what kinds and qualities of gear you find, and where you find it. Having the same droll and underwhelming uniques in the same exact place every playthough, and seldom being able to find any item in the world that is better than what you can craft or buy in town, pretty much removes looting and item hunting from the game, as a replay motivator. There is NEVER any feeling of "ok, lets clean out this dungeon and see what kinda cool stuff might be down there, and maybe we'll get an upgrade this time!"
2) Fix crafting and vendor itemization so that you can't craft or buy stuff that is always better than what you can find or loot or quest for out in the world. As it exists now, these two things make getting the best items in the game too easy, and only exacerbate the problem in #1. Finding the best items in the game should always be something you have to at least leave town for, IMO. If a crafted item is going to be as good as any of the best items in the wild, then please make it *extremely* difficult to make- long, challenging quests to get the pattern/recipe and some key components, can't just buy most of the ingredients in town, must have maxed-out craft skill, that sort of stuff. The way it is now adds nothing to my desire to replay, or even just continue playing.
3) Some semblance of gameplay balancing, with a proper 'Master' difficulty, that actually provides a reasonable amount of difficulty to ANY player, even those who've availed themselves of the better items and gear the game readily has to offer, and the best skill and perk combinations available to all. Why do so many people quit their characters by the time they get to level 25 or 30? Why do so few players try to max out their characters? While there isn't any one reason that applies to all, I suspect this lack of challenge and difficulty once past the starter levels, is a rather big factor in that loss of desire to play farther, and eventually weaken's a player's desire to continue playing the game. By the time you build up a character to where it is starting to be strong, you've already beaten the game, and removed the challenge. Even if it's only level 25. Instead of having varying levels of 'strong'... getting strong at 25... getting stronger at 35, really strong a 45, and 'as strong as almost the toughest monsters' at 55 but still finding challenging combat from the hardest monsters... no, you just hit "ok, i'm 30 and I cream everything now, game over, man". Something's wrong, there. Lack of balancing-1. Replayability- 0. Fixing #1 and #2 will go a long ways towards fixing this too, of course.
4) Better written and more good quality quests, less emphasis on radioactive quests. Quests and quest chains with different dialogue choices, different options, different things to do depending on which choice you make, tangents, results that affect your character, your character's standing with multiple factions, how people and factions react/respond to you due to which choices and actions you have made... and lasting affects on the gameworld and the NPC's in it in response to your choices and actions. Stuff other companies have done well, so I know it can be done. This would freakin' HUMONGOUSLY improve replayability and longevity, and the desire to replay the game in many different ways in order to explore all the different quest possibilities. But, no matter what you do, or how you play, the quests will always be 100% the same, and utterly friggin' replayability-killing boring.
I'd add on some more things that also detract from replayability, but my WOT (wall of text) quotient is already redlining.