There was no big legal battle between Obsidian and Bethesda. It was between Interplay and Bethesda and Bethesda won. Bethesda has the full rights to all things Fallout. Bethesda hired Obsidian to make New Vegas. Obsidian is made up of alot of people that worked on the orginal Fallout's and the cancelled Fallout 3 aka Van Buren.
Fallout New Vegas isn't an addition to Fallout 3. New Vegas is it's own game and has next to nothing to do with Fallout 3. New Vegas is more of a unofficial sequel to Fallout 2. Fallout 2 is the direct sequel to Fallout. Fallout 3 despite the 3 next to it's name isn't a direct sequel to Fallout 2.
I hope one day Bethesda hires Obsidian again to make another game. Also I suggest you play Fallout, Fallout 2 and Fallout Tactics
Edit: Fallout 4 (any future Fallout game) suggestions, ideas and speculations go here => http://www.gamesas.com/topic/1414088-fallout-4-speculation-suggestions-and-ideas-thread-90/