Chris Avellone has recently replied to a question on Twitter: "@ChrisAvellone Are we going to be allowed to return to Zion in HH after we beat it or is this gonna be like Dead Money (please god say no)". Chris then replied:
This gives the answer to the question that has concerned many Fallout Wiki users. This does not say that gameplay extends passed the Second Battle of Hoover Dam, but states that returning to the lands after completing the next add-ons will be possible.
Avellone also tweeted:
This confirms that lobotomites, which were originally to appear in Fallout 2, but were cut from the final version of the game, will appear in one of the upcoming add-ons (most likely Old World Blues).