I'm not good with dates in Elder Scrolls lore so that could blow my plan apart. Anyway the jungle in Cyroddil is from Imperial guide to Tamriel and is explained in Oblivion that Tiber Septim replaced it with forestry. So to get to the point: Elder Scrolls Online could be set before some incident that caused Cyrodiil to be changed into jungle. Not only those this explanation make Cyrodiil fit into lore without changing anything, that already had a lot of effort put into it. It also could set up for a expansion pack, similar to the Cataclysm expansion pack for World of Warcraft. Cyrodiil will be changed adding new PvP options (ambushes?), instances and new content.
For my personal idea: It could be something to do with ancient Ayleid magic (It is Cyrodiil after all). I was thinking maybe they excavate a large Ayleid city and find a powerful source of magic, they begin to bring it to somewhere to check it out when maybe they're attacked (Cyrodiil is war-zone after all) and somehow activate the artifact which is later found out to fertilize entire provinces, the Ayleids were meant to use it on Hammerfell but never got the chance. As well as changing PvP in the zone (new content), it could add either a raid or new area with the Ayleid city.
That's just my personal opinion on the matter and I hope Zenimax Online do a good job anyway.