So I just finished the game with the wildcard ending, and now I need to do the hoover dam mission again after the end game? Like there is a menu if I press no i'll be returning to the menu and if I say yes it loads a safe file before hover dam... what happens..

The game ends at Hoover Dam chum. There is no post MQ content, as most of the endings would require a large portion of the content be altered.

EDIT: accidentally'd some letters.

Yes your game will end after Hoover Dam.

Even if you have all the DLC added, it will still end.

Hearing this question over and over again can get pretty tiresome...

Yes, so save before you go to the battle and you can just do what you want like side quests and the DLCs.