A while back I came across a leaked list of area names that may or may not be real (but for the fun of speculation let;s assume they are) - they are as follows:
[spoiler]
Cave Crevice, Souldrift Cave, Madula, Palace, Altar of Fate, Forest of Rotted Giants, Cradle of Amana, Old Forest, Pyroxene District Jeldora, An Diel's Mansion, Prison Castle of Oblivion, Underground Water Prison, Belltower of the Moon, Collecting Valley, Tower of Earth, Hidden Port, Fused Iron Castle, Triple Bell Tower, Hunter's Copse, Arena, Undead Execution Chamber, Undead Citadel, Kuzu Bottom, Black Canyon, Doran Castle Lower District, Guardian Dragon's Nest, Ritual Chamber, Flame Successor's Throne, Valley of the Underground Palace, Haide Flame Spire, Blue Temple, Forest of the Hollow's Shadow, Gateway of Faros, Saint Graveyard, Memory of Giant Wamuda, Memory of Giant Ojei, Memory of Giant Roh, Altar of Amana, Castle Dorangregg, [b]King's Corridor, Undead Mausoleum,[/b] Old Dark Hole[/spoiler]
Bolded the ones I think will include the final boss - i'd say it's fair to say that the final boss will be someone who linked the flame/tried to link the flame, or has obtained some sort of power related to the undead curse.
[spoiler]So, Solaire linked the flame and found his sun, calling it now[/spoiler]
An epicly huge, gigantic wheel skeleton, that spawns more wheel skeletons as it rolls. You cannot block the gigantic horror, as the sheer weight and mass of it is too much. His name would be Aecsended Nito.