I'm going to post this because I think it best shows the pattern I see. Note that this post is from the future (
) where TESV has already been released.
I recommend ignoring Fallout 3's section until you see the pattern with everything else, because Fallout 3's development is a little weird due to the fact that it was "announced" when Bethesda bought the rights to it but hadn't even started developing it yet. With everything else you can see that the announcement is in the middle, while Fallout 3's is in the beginning.
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Work started on Morrowind in November 1998, it was developed all of 1999, announced in March 2000, shown at E3 in May 2001, and released May 2002.
(1998, 1999, 2000, 2001, 2002)The team split into two teams, one developed Morrowind DLC and the other developed Oblivion until June 2003.
Work started on Oblivion in May 2002, developed in 2003, announced in September 2004, shown at E3 in May 2005, and released March 2006.
(2002, 2003, 2004, 2005, 2006)The team split into two teams, one developed Oblivion DLC and the other developed Fallout 3 until October 2007.
Work started on Fallout 3 in July 2004, developed in 2005, developed in 2006, the first trailer was released in 2007, it was shown at E3 in July 2008, and released October 2008.
(2004, 2005, 2006, 2007, 2008)The team split into two teams, one developed Fallout 3 DLC and the other developed TESV until August 2009.
Work started on TESV in October 2008, developed in 2009, announced in 2010, shown at E3 2011 and released in 2012.
(2008, 2009, 2010, 2011, 2012)================================================================================
That's just what I think will happen anyway. Thoughts, opinions?