Well for one, each group should have different looks depending on the location, same with weapons, hitpoints, etc...
Junktown Raiders would be easier then New Reno Raiders, San Fran Raiders harder then Broken Hills Raiders, etc...
Junktown Raiders have more spears, Hub Raiders/ Boneyard Raiders have more gun guys, Broken Hills Raiders have more numbers and a few Ian looking guys with Smg's, or a Baldy in LA with a shotgun. Tougher Raider groups like San Fran Raiders, Modoc, etc have more hitpoints and a 10mm or .44 instead of a 9mm...etc
There should technically be two or more "groups" of Raiders, there's the "Khans" "Vipers" (Cancelled Raider Group) and the "Jackals" (Cancelled Raider Group)
http://fallout.wikia.com/wiki/Raiders(The player should have the chance to join any of the three for their SG Profession, or for w.e other reason
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The Khans could be the current Raiders faction that is used for the Sg profession, the weakest Raiders as seen in the wasteland with 9mm Mausers could be the Jackals, and the tougher Raiders with combat shotguns, assault rifles, etc could be the Vipers...
We can distinguish each group by their numbers, weapons, colours, npc models, etc...even give them their own reputation slot...
The current system works good so let's not remake it, but sometimes Raiders should carry more then just a 9mm Mauser, there's plenty of low tier guns they could be using, like the Deagle or the underated .44 magnum. So far only the Slaver's use Deagles, and almost nobody carries a .44 magnum. Sawed-off shotguns could be a signature weapon for Raiders too, since it is a pretty "lawless" weapon. It would be exciting to see random weapons on Raiders and go hunting for different groups of Raiders on the WM.
Low level players could hunt around Raiders around JT, then head to Necropolis for the slightly tougher Raiders (they get tougher the further you are from civilization) then head to Broken Hills/ Vault city for the even harder Raiders, then head to Redding/ Modoc for the next level, then finally San Fran for the bad-ass Raiders.