First horsebox at stables whiterun , look for trapdoor,  Here is a basement with loads of extra's
extra exits in Whiterun and Solitude. Or in the Ragged Flagon.
one in the center of Tamriel ( console command = cow tamriel 0 0 ) or use  "bleakwind bassin" marker.
An portal made in the tempel of Dibella, or in QA smoke.... what ever
Just for fun.  2 followers, one inside and one outside the stables ( twins ) , follower friendly,  navmeshed
a lot to take away and a few hidden rooms to find..........

test it maybe you are surprised what it has to offer...........