I would like the game to have mature themes. It could make the game feel more realistic, and would allow it to be more correct lorewise.
There should be poverty, violence, drugs, and other nasty stuff in the game. The cities should be larger, dirtier, and have more people. There should be poor districts, and rich districts in the cities.
The taverns in TES should feel more alive, and be a lot more noisy and uncivilized than in the previous games. It will really add to the immersion... the taverns in Morrowind and Oblivion really svcked.
People should also have have different shapes and sizes (maybe have some body-sliders in addition to the face-gen?), that could make NPCs look unique.
I'd like people to still have permanent underwear, because I don't want a lot of nvde dead people lying around after I've killed them and looted them.
I guess nudity in the right context could be okay as well... if the TES lore requires it. I understand it was done in Daggerfall.
I'd also like relationships with NPCs...
You should also be able to befriend NPCs... unique dialogue options, and other options that makes that NPC actually seem like a friend. That would really add to the role-playing experience. Having NPC friends was something a missed in the earlier games when role-playing.
Some NPCs should also be romanceable. I don't think six is needed though... but your character should be able to kiss his/her lover. six cutscenes like in ME or DA:O would not break the game, IMO... but it's really not a necessary feature.
I don't see any point in going much deeper than that. Modders could probably expand this system if they wanted to.
Concerning the blood and gore, I'd like a realistic amount. Not like in FO3. Some spells could probably make an ugly mess out of people, but not a normal weapon... at least not unenchanted ones.
If TES5 got an AO-rating I'd buy it anyway... but I find it unlikely to happen. I'm expecting and M-rating.