The Elder Scrolls V
There is speculation that the next game could be based in Skyrim. The name has been trademarked by Bethesda, which could indicate that there is an upcoming game called Skyrim, as there was for Knights of the Nine and Shivering Isles. A simple search at the US Trademarks website reveals four applications for Skyrim as a Trademark, three from Bethesda, and one from a telecommunications company in Florida. Bethesda filed for the trademark in October 2006. It is interesting to note that Bethesda filed for Oblivion's trademark on June 21, 1995, but abandoned the trademark, only to refile for it on April 4, 2003 about 3 years before release (2006). Morrowind followed a similar 3 year pattern (1999 to 2002). It is unlikely that Skyrim will follow this same three year release, since it has yet to be announced. Also Michael Kirkbride, a contract developer for Bethesda, is rumored to be working on Nordic lore with text such as The Seven Fights of The Aldudagga.
There is some speculation that TES V will take place in Summerset Isle, due to various rumors about Daedra Worship increasing and a boycott of Imperial goods in Oblivion (although it is important to note that there are several equally 'significant' rumors about events in other provinces in Oblivion, namely Black Marsh, Morrowind, Orsinium, Elsweyr and, of course, Skyrim). It is possible this could follow the pattern set in Morrowind, which had rumors mentioning trouble in Cyrodiil. But of course, this logic also holds true for all the other provinces mentioned. (And again, it is worth noting that none of the events mentioned in rumors about Cyrodiil in Morrowind actually occurred in Oblivion; only those prophecies which spoke of general plot events, namely a Daedric invasion, not specific to Cyrodiil)
In fact, it was rumored that TES IV would take place on Summerset Isle, due to developer comments from before Morrowind's release, but those proved to be untrue.
In terms of gameplay, Summerset Isle would be a convenient choice because it is surrounded by water, providing a natural boundary to the playable game-world, much like Morrowind had by taking place in Vvardenfell and not the mainland area of the province. In Oblivion, the invisible walls at the edges of the mostly land-locked Cyrodiil were a cause for complaints by some fans.
The events of TES V may take place 200 years after the events of Oblivion. A 1up.com post states that the first Elder Scrolls novel "partly bridges the gap for the next game, which is set 200 years after the Oblivion crisis." The retail site from which this information came quickly removed the reference to the next Elder Scrolls game from its listing; it is unknown whether this is due to a factual error or if the retail site accidentally leaked information that was meant to be released at a later date.
It is speculated that Bethesda started full development of The Elder Scrolls V after the completion of Fallout 3. From previous quotes from Todd Howard, it is suspected that preproduction work for TESV took place during Fallout's development. At one point there was news that pointed towards a 2010 release date; however, that appears to be false, as the game has not been announced, and a 2010 release date would leave little time for promotion of the game. The 2010 date was always regarded as questionable, since Fallout 3 used the same basic Engine as Oblivion and still required 31 months of development time (The 2010 date gave a maximum of 26 months). It is important to note that Bethesda has announced that there will be no more downloadable content for Fallout 3, which indicates that Bethesda is increasingly focusing their developers on their next stand alone game release.
On April 27, 2009, Pete Hines was quoted by IGN as saying that no new Elder Scrolls title was expected to be announced "anytime soon." This was done along the announcement of the Elder Scrolls Novels to be penned by renowned fantasy author Greg Keyes. The announcement is ambiguous and no more specific details were given. The interview can be found here.
In a more recent interview for Gamasutra dated June 25th, Hines let slip that "On the flip side, Todd [Howard] and his team have their hands full with The Elder Scrolls and Fallout." This suggests that some Elder Scrolls title, be it Elder Scrolls V or a spin-off along the vein of Redguard and Battlespire, is in production on some level. That interview can be found here.