For Yes Man I'm not even sure, the ending was quite vague too me. Yes Man says he'll become more aggresive but I don't think that means he kills the courier and takes over. At the same time as Oliver said it's a large responsibility to start your own state. Could you really do it? Honestly I felt the Yes Man ending was played down by the fact that no matter what the Courier was ineffective and let anarchy rain for years after his rule. Here's my examples for each faction.
Yes Man ending
First thing first send a fruit basket to the NCR for throwing Oliver off the Dam lmao but seriously there's some real work to be done.
A. Block off the Mojave Outpost kick the NCR while it's down.
When the NCR decides to come back I'd want to make things as hard as possible for them that said some C-4 on those large stautes and block the route with debry. After that post some securtons there and try to mointor NCR actions. I doubt that Kimball would survive re-election if he's not already dead from the assassination attempt and even if Oliver's alive it's not as if he's effective. The NCR senate would pull support for further operations in the Mojave and they'd experience a Vietnam type sydrome. I don't even think the Baja campaign would be continued though who knows how large that is.
In the aftermath of the Second Battle I'd kill as many NCR troops as possible, and any that survived at Mccaran from the Fiends would be easy targets from Securtons. Any at Golf would of died from the Legion, the ones at Forlorn could be taken care of. After that the remants would try to retreat through Mojave Outpost would be killed off by lack of supplies, raiders, or securtons.
B. Legion no more
If your going indepedent I don't see the logic in keeping Legate or Casear alive. For this purpose lets assume you killed them both. Now add in the Veterens who died in the Second Battle, now add those who would die from securtons cleaning up. Without leadership I doubt the Legion would have the resources to return. Hell talk to Legate he even knows some generals back in New Mexico and Arizona aren't loyal and would set up their own Despotic State. Honeslty besides Cottonwood I wouldn't even be concerned about the few Legion alive.
C. Other players
Rally the tribes as soon as possible and keep order in the Strip and surrounding Slums. Go to the boomers yourself and negotage trade relations. The Kings would be able to keep Freeside safe and the Fiends would be largely dead from the attack on Mccaran so I doubt they'd been too hard to deal with. The great khans aren't crazy but might as well be killed. Jacobstown isn't worth the time. There are many long term things to do and I'll probably go into great detail about those later. Hell I would of killed off the BOS since they'd try to control I-15 and Highway 93. That's no good for trade.
I'll make up a Legion one later, but for now what would you do to expand your factions empire?