Creating Tamriel wouldn't be that difficult actually, especially with the help of the new land editor. The difficult part is a) recreating important landmarks such as Dagoth Ur, Crystal Tower, etc,

adding well over 10,000 new custom NPCs to the new land, c) Reanimating textures for guars, cliffrunners, and other creatures from outside Cyrodil, and d) Adding the quests to each area (if Cyrodil alone has 150 quests, multiplied by eight = 1200 new quests you'd need to add).
The whole project would require over a year to finish and a huge group, but it's very possible.