Tales of the Grotesque and Dungeonesque has tackled the question, “What makes a monster interesting?” The answer provided is the old standby of solutions to the been-there-done-that doldrums: reskinning.
The problem with reskinning is that it's just kicking the can down the road. You've made a boring monster more interesting for a single encounter. What about the next time? And the time after that? You could just use different monsters every time, though if you're going to do that, why not just use different monsters before resorting to reskinning? There's almost certainly a goblin-analogue in Fiend Folio you haven't used yet, like xvarts or dark creepers.
What really makes a monster interesting is what the players can do with it. If your “wizened, man-like fey” are just another EXP-piñata, well, ok, the PCs attack, dice are rolled, moving along. On the other hand, if the PCs can confound them by wearing their clothes inside out, that's a bit more interesting. Goblins you can trade with are more interesting yet, especially if they allow you to push deeper into the hex-crawl or are the only source for certain goods.
I want to return to that “inside-out” thing, though. Monsters that invoke fairy-tale logic are some of the best because they prod the players to interact with the world in non-standard ways. Vampires are awesome for this because they're nearly impossible to kill otherwise. But everyone knows how vulnerable they are if you expose them to sunlight or find their coffins. Now, suddenly, all sorts of things about the adventure are important: where is the nearest holy ground, what time of day is it, do the PCs encounter the vampire deep underground or in a tottering ruin or at a public event where exposure could thwart its plans? Players who couldn't care less about the campaign's calendar are suddenly very interested in the phases of the moon when they know they're up against lycanthropes.
Finally, monsters are interesting when they have a noticeable impact on the world. Goblins hiding up in their caves are not terribly interesting. Goblins who are raiding merchant caravans and driving up prices are a lot more interesting. Goblins who have infiltrated a walled city's sewers and are stealing babies for some nefarious purpose are more interesting yet. And they get even more interesting when they're feeding those babies to a black dragon who will rise from the sewers and wreak havoc should the flow of babies be interrupted by, say, a group of do-gooding adventurers. When slaying the monster doesn't mean just additional EXP, but also affects the world around them (lower prices at the blacksmith or the gratitude of a city no longer on the verge of riots), that makes the whole world more interesting. That's one of the cool things about dragons in the old stories. Slaying a dragon wasn't just an extra notch on the knight's sword hilt. It meant a new lease on life for the entire community the dragon was preying upon, it meant a happy reunion for the princess and her family, it unleashed a flood of lost wealth returned to the local economy. 5e kinda gets at that with their regional effects for “mythic” monsters, one of the things I very much appreciate about the new edition.