How do you keep playing after you beat Fallout 3

Fallout 3 Once you've beat Fallout 3, you can simply go to New Game and start a new quest. With over 200 possible endings, there are many different scenarios.
The only way is to wait until April and buy the Broken Steel Add-on from XBL which allows you to play more and get lvl 30.
After u finish main quest just load D.L.P DISK then load game from main disk an ur pipboy will up-date at the last seen in the game b4 u get in the chamber with lyons don't finish the mission an go back to megaton and sleep then you can play all the missons you missed an the new ones.
FALLOUT RULES!!
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There was a way I did it, if im sure. I had all the downloadable contents from the market place:^
Broken Steel
Mothership Zeta
Operation Anchorage
Point Look Out
The Pit

When you die in the gas chamber it plays the video what happend after you die. Then you start to wake up and the downloadable content show up on your screen saying you have it while you get up. Now you can continue playing Fallout 3. I am not 100% sure but that's what happend to me.