I know there are billions of reasons to not make it online but there are also billions of reasons to make it online, so why not make it an online game?
If enough time and effort can be put into the next series (if there will be any) it could be implanted as an online roleplaying game, the first of its kind and im betting millions of players would be playing it.
Sorry, but the single player aspect is the only reason I play TES games. My character has a bigger impact over the game world than he can in an online game where everything is static and never changes. Online games take away freedom and often cater to the lowest common denominator type player.
If you want an online game then go play one, there are 100's to choose from. TES is unique because it offers a single player experience that is unmatched and gives me the freedom I enjoy. The freedom to kill who I want, the freedom to steal stuff from whoever I want, the freedom to loot whatever I want, the freedom to change the gameworld based on my actions alone, etc. Online games will never offer this.