Who says that in the next add-on they should add a time machine and it will let you redo one or two quests? It would help a lot for some people who don't have the PC version. Post your comments and we'll see who wins.
The DLCs are set in stone, so this won't happen.
Time travel in a Fallout 4 DLC (though not for the reasons you mention) could be interesting, though. A localized time experiment, seeing the same locations pre- and post-war, optimistic time travelers landing in the future apocalypse... this could be a fun DLC if the proper tone was maintained.
But not in New Vegas. Post this again in a couple years.