Okay, so, I have learnt about what appears to be the setting for some DLC content regarding Skyrim.
Travel to Stendarr's Beacon and head a little bit north.
You will come to a stone archway. Just before you reach it, it will say "You Cannot Pass This Way".
Given that this archway is open, and no such archway is mentioned between Skyrim and Morrowind (which is to the East of Skyrim), this is likely going to feature DLC content regarding skyrim.
I tried it, and couldnt get past it, even with DLC content, however, if you fly up high enough, you can clearly see Red Mountain, so they obviously put a lot of effort into including those locations. So... yeah, it may be that we see these in the near future.
p.s. the site seems to be posting incorrect information about being able to actually travel into Morrowind, however its quite possible that this may be possible in an expansion.
BLOGS AND VIDEOS SHOWING "DETAILED" SECTIONS OF MORROWIND IN THE CURRENT FORM OF THE GAME ARE FAKED