the first expansion, instead of being some tiny little place, should be another full zone, like morrowind, with a completely new main quest, guilds etc, etc. and when starting a new game, the option to start the morrowind zone before cyrodil.
then another one with another place like skyrim, and continued until the whole of tamriel has been done.
preferably each of the expansions should be able to work in conjunctions with any combgination of others and each one should be playable as a standalone game.
admittedly rather far fetched idea and the amount of hard disk space taken would be in the 30gb area for it all, but what i'd like to see.
it would also allow each one to be replaced individually when the graphics or gameplay go out of date..