Problem with Raider Kingdom is that New Vegas raiders have so much different goals that they couldn't unite. They even can't even cooperate on their own.
Powder Gangers:
-Joe Cobb at first wants to shake down Goodsprings, but if you help him with Sunny and other defenders, he settles in and starts providing protection to town
-Eddie wants to secure his power and rulle alone, to achieve this he even asks you to elimiminate fellow Powder Ganger who wants some indenpendence from him (Chavez)
-Chavez wants to earn quick money by raiding nearby caravans and move on with his life
-Samuel Cooke wants to kill NCR
-Philip Lem wants to surrender to NCR
-Meyers can become a sheriff (cut content includes some other Powder Gangers becoming his deputies)
-Boxcars, Swanick and two Nipton hostages just bail out of it
Jackals:
-small groups of bandits armed with melee weapons, 10 mm pistols and cowboy repeaters and dressed in rags
Vipers:
-they ambush prospectors with machetes, sledgehammers and occasional 10 mm. They use armours in poor condition
Fiends:
-bunch of drug addicts, that haven't got crashed by NCR only thanks to Legion and their huge number of Fiends
-their leaders are nuts and act chaotic, which leads to high casualties in their ranks
-Cook-Cook goes frenzy if you kill his favourite brahmin, he's also rapist and sadist
-Driver Nephi rushes into battle with his golf club
-Violet is surrounded only by her dogs and they pose no threat to someone with submachime gun, ripper or Animal Friend
-Motor-Runner is the most intelligent of them all, but he's still a bad strategist. His hideout is a mass grave with only one entrance. Also his personal bodyguards are two dogs
-Fiends lose quickly to the NCR, if you kill their leaders (which isn't hard)
Freeside thugs:
-bunch of poorly armed muggers, that get killed quickly by Kings
Kings:
-they don't care about anything that isn't related to Freeside
Great Khans:
-they are the only gang that has interactions with other gangs (and they have them because of their mutual hatred towards NCR)- they sell drugs to Fiends and Papa Khan can be asked to let Samuel Cooke and his group join the Khans
-they aren't ruthless fighters
-Papa Khan and Regis can be easily persuaded to change their plan to join Legion- you can even convince them to try to forgive NCR
Strip Families:
-their main source of income are gamblers and tourists
-even all three families members together wouldn't pose much a threat
Quick summary. There isn't any chance for them to form a great raider alliance. Even if we considered Cooke- Fiends- Khans alliance it wouldn't survive long. I don't think it would even survive to the fall of NCR, but if it happened it would be the end of alliance.
Only three gangs in Mojave have something in common- hatred for NCR. Without the NCR it wouldn't survive.