How about a nice big PvP event? 
Yes because everyone loves those....
Well, how about this:
"The front lines are everywhere........."
You arrive at V-15 buck assed naked. You are asked which faction you want to serve: The options are BoS, NCR, VC, Free Merchants, Boneyard Scavengers, The Families, Necropolis/Gecko, Redding, Klamath, and Enclave (maybe others, I leave that to devs/gms to figure out).
Pick your faction, then be kitted out; Body armor appropriate to the faction, choice of weapons appropriate to the faction, other gear as appropriate. (Kinda like the PvE event of defending SAD a few months ago, but more efficient.) You are then thrown into battle; it works kinda like a town control, that is you're fighting on a replica of the other faction's territory, but you're fighting other players that chose those factions (ie, if you join the Enclave, you would be sent to a replica of Lost Hills to fight BoS-allied players). If you get killed, you end up in a regen point.
You COULD choose to hump back to V-15 and re-join the war (in the same faction or a different one), or just quit. Each battle you fight makes your rep with that faction grow just a little bit, until you could (at least in theory) become "Liked" or better, at which point you'll actually be allowed to enter even certain restricted parts or the faction (if there are any) without being killed for standing there (for example if you join with the Enclave, you can talk to their quartermaster).
The war continues until you've fought against every other faction. After the war, the other faction's won't hold it against you (no effect to rep), and factions stratecically allied with the factions you join (again, you join the BoS, their strategic allies include the NCR and to a lesser extent the Gun Runners) will actually award you for your service with 1/10th the rep you picked up from the faction you did join.
