It greatly depends on what platform you are using.
IMPORTANT: To get any DLC at all, you will need a working internet connection.
The DLC in Fallout New Vegas allows you to do more missions, which means yes, it keeps you playing.
No no more DLC is being developed for Fallout New Vegas. There are; however, multiple Area 51 mods you could take a look at.
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Gun Runner's Arsenal.
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The Sierra Madre is the location of the Fallout New Vegas DLC Dead Money. This DLC cannot be bought anywhere on console other than live.
No obsidion said they would not release dlc that extends gameplay past the ending
All of them except for the "Dead Money" DLC.
You need to have the 'Lonesome Road' DLC installed, the Canyon Wreckage is the starting point for the DLC.
Yes, you can use weapons from Fallout New Vegas DLCs in the base game.
The unique gatling laser in Fallout New Vegas (The Sprtel-Wood 9700) can be purchased from the Gun Runners provided that you have the Gun Runner's Arsenal DLC.
Ulysses only appears in person in the final DLC pack 'Lonesome Road'.