as to who the new Emperor of Tamriel is. Martin dies...so who takes control? PS...I never even came near to finishing Oblivion, so I have no idea...

I suppose The Elder Council is takeing over Tamriel for the time being, till they find another Emperor.

http://www.imperial-library.info/content/fourth-era

Fixed a screwy link.

As of the infernal city http://www.uesp.net/wiki/Lore:Titus_Mede#Titus_Mede is the current emperor of Tamriel. Although the "empire" now barely even controls most of Cyrodiil.

A while ago (a year, maybe?), an Elder Scrolls novel titled "the Infernal City" was released. It takes place ~50 years after Oblivion, and reveals that, with the end of the Septim line, the Empire essentially fell apart. An unspecified time after that, a warlord named Titus Mede managed to reunite Cyrodiil under his rule, and crown himself Emperor, although all the other provinces are still independent.

Of course, we don't know yet when Skyrim will take place, in comparison to the book, so it's unknown how relevant this will be to the game.