Co-op would need to be carefully implemented; here's an idea:
A guild which can either be completed singularly so as to not detract from those who play without Live / prefer to hardcore role play in games like Skyrim, or could be completed cooperatively with a friend. The core storyline will remain relatively the same (a similar end-goal and similar expository situation), while the means of reaching the end of the storyline alter according to which one you go through.
Perhaps this co-op guild could also have a minimum / maximum level limit so as to prevent any great disparities between levels of one player. For example, the guild for co-op could only be completed at levels 25-35 (or at any level for single player), thus making the enemies still tough and requiring teamwork, but still defeatable for cooperative play.
Co-op would be limited to this questline and its associated areas only, which would include a town, some dungeons and a few quests for people to complete after the main guild story so as to make co-op a more ongoing thing instead of a one-time-only deal, as well as hopefully preventing any interference in a single-player story.
The most important thing is that co-op would happen on an invite-only basis. People who have this gloom-and-doom prediction of people joining your world only to wreak havoc are stupid -- I highly doubt a reputable developer like Bethesda would allow such a thing.
Now, I understand the hardcore TES fan's appeal: "But they won't focus as much on SP!"
Well, honestly, as a previous poster said, a co-op questline would be farrrrr more interesting than all the radiant quests which add little-to-nothing to the lore or gameplay experience. I'd rather have a decent five-ten hour co-op guild questline with a little area devoted to co-op play than doing the same quest x number of times.
Besides, think of all the possibilities! Spells that require two people to cast, co-op-specific combat roles or attacks or equipment, imagine solving puzzles or clearing dungeons that may require two people! It adds a whole new level of depth and complexity to the core gameplay that Skyrim is sometimes lacking.
Most importantly, though, is even if it's incorporated... you don't have to play it if you don't want to.
"We Do Not Sow; Hear Me Roar; Unbowed, Unbent, Unbroken; Growing Strong; Ours is the Fury; As High as Honor; Night gathers, my watch begins."
can honestly say that every multiplayer game i played (not specifically competitive or co-op though i prefer co-op) was more enjoyable to me then any single player game ive played for the simple reason that video games are more fun when you play them with ur friends