The Chronicles of Tamriel
Mods like "Tamriel Rebuilt" and "Morroblivion" are generally very popular due to people wanting to paly in the rest of Tamriel rather than being confined to just a few areas. Bethesda could take advantage of this by episodically making a huge game that develops into a playable Tamriel. It could be along the lines of this:
Episode One - Valenwood & Elsweyr
Episode Two - Black Marsh & Mainland Morrowind
Episode Three - Skyrim & High Rock
Episode Four - Hammerfell & Summerset Isle
Episode Five - Vvardenfell & Cyrodiil
Episode Six - Massive content addition - requires all previous episodes. Adds a new main quest that will see you travelling all across Tamriel. New areas and quests in each previous areas. Could have some really cool features such as finding an old Frostcrag Spire, or having people reminice (did I spell that right?) about the respective heroes of each game (or maybe include these features in the original episodes)
Each episode includes a main quest. Notice that (almost) every pair of locations are next to each other for easy transportation, but there are loads of travel links (teleportation, silt striders, boats, levitation, NO FAST TRAVEL! >:[ )
Also, it should be noted that these are NOT seperate games; it works like the Morrowind and Oblivion Data Files system. This game would be great for modding, and Bethesda can really get it right this time. They know for sure what the community does and doesn't like.
So, whaddaya think?