There are a couple of mods I had used in Oblivion and Morrowind that I'd like to see return to Skyrim (and I'm sure some of you would agree)

1. Deadlier Traps
I love how Skyrim has upped the number of trap-laden dungeons and ruins, but so far, traps have proved to be a little weak (except that one swinging blade corridor that sliced Faendal to ribbons). There was a mod in Oblivion that made traps absolutely deadly and required guile to move around

2. Backpacks and Scabbards
Both these mods were available for the 2 previous games, but there was a particular mod in Morrowind that provided a hefty rucksack (the length of a 2-handed weapon) that made journeying and RPing more believable. I know with the decrease of armor/clothing slots this would prove a little more difficult to implement, but it would be a great addition. As for scabbards, you know the drill