Vault Dweller: Good karma for stopping the Master's army, game-wise he probably wore Tb-51 Power Armor and utilized Energy weapons as those were the best, a dog and a human friend were his constant companions, though both died, leaving him alone
Chosen One: Good karma for stopping the Enclave, game-wise he probably wore Enclave advanced power armor and utilized energy weapons, as those were the best. Had various companions, not sure if any were constant to him (Marcus and Cassidy obviously left at some point)
Lone Wanderer: Good karma for stopping the Enclave, game-wise he probably wore Tb-51 Power armor and utilized Energy Weapons, as those were the best. A dog was probably his only constant companion, as implied by the ending slides
The Courier: Neutral karma for simply choosing the philosophy and nation he believes in and it's impossible to please everyone by doing so and he MUST get blood on his hands at some point in the game. Game-wise he probably utilized light armor and cowboy weaponry (lever-action rifles and revolvers) as those were some of the best and the cowboy weaponry fit the theme. Had an eyebot as a constant companion, as implied by ED-E's game ending.
See wut I did thar? I found the Courier specifically to be SOOOO refreshing because he broke the mold and did something completely different, going into battle with a lever-action rifle and a duster while the other three guys have their power armor and gatling lasers. I enjoy some variety in what's the strongest setup and what direction the main quest takes you in, because to me, it somehow makes the Fallout world feel more alive. It justifies the use of normal clothing or light armor instead of combat or metal armor by all those random guys I find in the wastes and it helps me feel more immersed and interested in the story when there isn't a ridiculous predictable pattern among the protagonists.
What direction would you like to see the next protagonist taken in? What kind of karma tone should the main quest have, what weapon or armor type should get some love in the next title, who should the protagonist hang out with, etc? Or hey, why not a female protagonist? We're due for one of those.