The Empire. Because he founded it, and wouldn't want it to crumble.
Tiber Septim is definitely not my favorite historical figure, but he'd probably unite Tamriel, and not tear apart the Empire, which would be the opposite of everything he has fought for in his life.
The Empire in TES V: Skyrim, isn't his Empire. It's the Mede Empire, not the Septim Empire. Like others have pointed out, Talos himself in the form of Wulf, said that the Empire was going to crumble.
Oddfrid White-Lip, the "talking skull" in the Bloodmoon expansion, also prophesized the fall of the Empire. It said
"when the dragon dies, the Empire dies". When TES IV: Oblivion came out, we thought he was specifically talking about when Akatosh appeared. But instead, could it been referring to Alduin as "the dragon"? Alluding to when after Alduin falls, the Empire will crumble as well? I believe so. The Empire is on life support, and the plug is about to be pulled.