Name three. Seriously, name three fantasy characters, or, heck, even with your caveat of historical fiction, characters that can be modeled well using a plain jane 3e fighter.
You mean guys like Charlemagne, Beowulf, Sigurd, Ivanhoe, etc.? What about Arn Magnusson?
Now, some of these guys I'd term "Cavaliers"......but that's not a class in core 3E......so, fighter.
Then there's Lancelot, Gawain, and several others of the Knights of the Round Table. Some, such as Galahad would be more like paladins....but Gawain wouldn't have been.....most of them were fighters or cavaliers, without a stitch of spellcasting ability.
Cuchulainn, I assume, would be a barbarian, rather than a fighter.
Myth is full of powerful warriors....
That having been said, I think the game lacks by not having a social class/nobility type aspect, knights, politics, and proper use of strongholds etc. This stuff really wasn't there in 3E, aside from optional supplements. I do think the inclusion of such would have helped to establish fighters as natural leaders. Back in 2nd Ed. it was the fighter who gained the most followers out of anybody. And that meant something. It was cool to have a castle and your own personal army. I kind of missed that.
Even so....unless the fighter is competing against a bard in diplomacy, he can hold his own against commoners. It's not likely he's going to be competing against lvl 15 commoners to command people. Thus, he might be c ompeting against commoners of lvls 1-4, and for that, his limited diplomacy ranks will likely do fine. And that's without even changing any of the rules.
Banshee