In most D&D worlds, players are constantly romping all over monster lairs. Any monster communities that have at least some moderately intelligent individuals will learn from being tromped time after time. Orcs and/or hobgoblins, in most fantasy, are cunning and capable fighters. Your players might get more enjoyment out of devious and sneaky NPCs than they would get out of a new NPC type.
Having NPC's (of whatever race/species) learn from the action can be very helpful. Try to make sure there are survivors to the PC's privations of villages, lairs and such... they will spread the word to their people. The smarter ones will begin setting out lookouts/scouts, building ambushes, etc.
If the PC's come back enough times they're eventually going to have some nasty surprises waiting for them. Also, some of those surviving NPCs can become leaders of factions bend on revenge -- making them likely candidates for antagonists that can begin driving sub-plot, maybe even major plot lines as they begin taking a personal hand in getting payback.
Always let the PC's actions help drive the story. Where have they been? Who have they met (particularly conflicted with)? Some tavern fight could yield relatives of a dead guy bend on killing the party. A merchant who feels horribly cheated because the group pawned off a cursed item on him and he's out a ton of cash getting rid of it. They make great foils the PC's rarely expect until the bit happens.
There's the opposite too. Very high Charisma characters can end up with "secret admirers" (some might become outright Stalkers) who do things to confuse them -- possibly even get the part in trouble in some ways (however unintentionally). If a party member saves someone's life... that person can become an unwanted "follower" or "henchman", insisting they owe a debt and are bent on paying it back no matter how much trouble it causes, etc.
The group's own actions can and should build their reputation in a number of ways.
