Not trying to be negative about the ideals, but the primary way that experience is gained or that your character is 'furthered' in the game is by combat. That's the heart of dnd, combat. I know the heart of FK is RP, but when it's based off dnd, it's RP that revolves around combat.I simply meant that the goal was not to just make fighters more powerful in the sense that they would be able to solo more easily, but rather to make them more interesting/useful.
When making a fighter do exactly what he/she is suppose to do, you're going to make soloing easier for that person. There's nothing wrong with someone being able to solo some quests, whether it's running errands for someone or rescuing a maiden from a horde of goblins.
If we were required to group up for every quest, this game would be very dull (personally speaking). There would be a lot of "Hey, group up with me and follow me, so I can get this quest done" and very little interaction. I like being able to wander off and go train by myself, regardless of class.
But back to the topic at hand: To make a fighter more unique/useful, you simply have to differentiate his abilities/traits from the other classes. By making him/her a competent fighter who has the possibility of dealing more dammage vs. other classes (straight up fighting), you make them more useful to others. Does that mean that they'd be able to take on more goblins at one time than others? You bet. That's their job.
As for armor and dammage reduction, I'm not a huge fan of dammage reduction, unless it's some super magical armor. Typically, for heavy/medium armor, a bonus to AC would be ideal or a stat bonus. For the real nice magical armor, make the magical effects very noticeable. As in: A glossy red breast plate has a Str bonus of +4. That's not unreasonable, I don't think, while combined with Boots of Hardiness of +2 Con.
Honestly, what I'm guessing in the future is that more and more magical items will start appearing in game with stronger magical effects. "Intermediate" difficulty quests will start to become easier, so new quests will be added which will have no mercy for anyone who tries to solo it. Grouping encouraged - RP required.