Most of this is serious, but there's some silly stuff thrown in at the end. Just because.
- Marriage: Skyrim had a ton of people you could marry, and treated same-sex marriage exactly the same as heterosexual marriage. Both these things were cool. What was less cool was how rushed and last-minute marriage felt. Only a small handful of potential spouses were somewhat developed (and those were because you had to complete a lengthy quest chain that involved them- Aela, Farkas, and Vilkas come to mind right now). Overall, though, the potential spouses had minimal personality, and there was absolutely no relationship development. I would've been perfectly okay with having half the potential spouses (or less) if it meant the ones that were in the game got fleshed out and your relationships with them actually felt like... well, relationships.
- On that note, when the Dovahkiin marries a character of the same gender, it's not treated as weird/abnormal/unusual/special/
different, yet there are no same-sex NPC couples. This is a bit strange, and almost makes it feel like the ability to marry a character of the same sex was a coding oversight, which is kinda insulting. Just... y'know, make it feel like my Altmer and her wife aren't the only gay couple in the whole world.
- On yet another related note, CHARACTER DEVELOPMENT. There are some characters that are fairly close to the Dovahkiin that have zero personality, even though they really feel like they should have at least something. Namely, the housecarls. They're your bodyguards and essentially your roommates, yet you don't get much out of them besides "long life to you, my thane." They have some very vague hints of personality (Lydia says "I am sworn to carry your burdens" sarcastically, Jordis gets excited if you take her on adventures, and Iona will lecture you about stealing and put a bounty on you if you do it in her presence), but overall... There's nothing. These are characters that live in your house with you, and yet you learn almost nothing about them. (Argis in particular irks me, he's scarred and blind in one eye, this is a guy who clearly has been through some bad stuff and has stories to tell, yet... nothing.)
- The Blades' story. Killing Paarthunax is the last task in it, and if you don't kill him you can still use your Blades recruits as followers, but if you do kill him then the Greybeards won't tell you where the next word is. Whether to kill him or not is supposed to be a difficult, gray-and-grey decision, but there's really no incentive to kill him since the Blades' story doesn't continue and their services as a faction are less useful than the Greybeards'.
- A third option for the civil war, where you get the Legion and Stormcloaks to make peace and start gearing up for the next inevitable war with the Thalmor, since neither side has any love for the Aldemeri Dominion. Possibly make it part of the Blades' story, since as mentioned above, there isn't a whole lot of incentive to pick them over the Greybeards. It'd make plot sense too, as Delphine was the one who had you raid the Thalmor embassy, and that's where you found the dossier on Ulfric...
- Speaking of that dossier, some way to confront Ulfric about its contents would've been cool, and would've raised some really interesting questions for a Stormcloak-allied Dovahkiin.
- A Daedric quest for Jyggalag, even if it's just something like "bring him 3 wolf pelts and then organize his bookshelf." Poor guy's even less popular at parties than Malacath, and Malacath is not popular at parties...
- Khajiit and Bosmer potential spouses, even if it was just one of each.
- Rikke and Ulfric as potential spouses upon completion of the civil war for the Legion and Stormcloaks, respectively. For the lulz.
- Curly hair, because it's cute.
- The ability to buy a pet dog instead of just hire one.
- The ability to dress up your spouse without having to fiddle around with console commands.