Having always had an interest in Arthurian stories, when I heard about BBC's Merlin I checked it out. And I was so disappointed in it that I didn't try again for several years.
One work that made me think I should was the vid
"Red" by obsessive24. It's striking to watch, even if one doesn't know the canon (and, as is the case with quite a few Merlin vids, makes it look much better and more coherent than it is). It was likely because of Red that I ended up trying out
Steam by new_kate, "Renaissance-era Steampunk reincarnation AU featuring brilliant engineer Prince Arthur, his savant mechanic Merlin, a robot bird, and an evil mecha."
I don't even recall how I came across it now, but I'm guessing it was someone's rec, and it sounded intriguing enough that I gave it a go. I enjoyed the fic and decided to try the series again before going after more fanworks.
I still think it is a deeply flawed canon, but after a bit I grasped why the fandom had latched on to it so tightly. That would be a separate essay, but without fanworks I would surely never have watched the show nor continue to read in it regularly.
In terms of minor characters, the first one that popped into my head was J.A.R.V.I.S. He adds a lot to many a story, but I recall one in particular focused on him --
The Son Of Man by copperbadge. "JARVIS did not want to be a real boy. He was quite happy being a building." It was particularly nice that it was a long fic as well!
I don't tend to read much under 5000 words much less 1000, but I did have one item in my bookmarks, also, as it turns out, featuring J.A.R.V.I.S.:
Verb: Buttling by redsrule1.