This idea was one of the first novel concepts I completed - I think it comes in as the second, if I count "The Cats of Mordue" (please don't ask) as a single project. Understandably, it was chock full of all the fantasy cliches you can name: Nelia, a reluctant heroine with great powers being chased by a power-hungry evil overlord, a retired great hero serving as her mentor, and I'm pretty sure there was a prophecy floating around in there, too. (The geography I laid out for the villain's stronghold looks a lot like Mordor. It makes me snicker now.)
Things took a turn for the more interesting (I think) when I introduced Ilidan, traveling companion, intended love interest ... and he just happened to be the villain's son. Alas, I was too green to see the opportunity the first time 'round, and he pretty much devolved into a stock villain.
I took a rewrite lap of this project surprisingly soon - I think this was during the period when I was deluded into the horrific idea that a "draft" of a novel had to be starting over from the beginning with new text. And I was already starting to get the feeling that my treatment of Ilidan was inauthentic, a cheat - and that I had better give his father some motivation as well. (Since I'd introduced telepathy into the setting, I decided to use telepathic monitoring as my bone of contention.)
I'm not sure how completely I realized it at the time and how much is hindsight, but something felt off. I looked at the ending, and it started to feel as if the most interesting choice in the story wasn't Nelia's, but rather Ilidan's. Hers was the "simple" heroic choice; his was more nuanced, more thorny. At the time, I didn't have the tools in my box to handle this. I just ignored it.
So the game plan to tackle this project again would be simple: give Nelia and Ilidan an equal share in the book. Make it *their* story rather than her story. That will change a lot of nuances; it may even change the ending. (Honestly, a lot of the plot and setting would be going in the blender anyhow, since I want to introduce some less hackneyed elements.) And here is where the almost-familiarity of the basic story becomes an advantage, because I can take something that readers know and are familiar with and re-cast it in a new light.
One of my huge concerns here, no surprise, is making it a project that an editor or agent will want to pick up and not immediately go, "I've read this a hundred times before." Part of this I can handle by re-casting the setting and plot, but I need the familiarity to pull off the conflict between the two characters. And the fact that this new construction almost certainly makes the book not only stand-alone, but necessarily sequel-less, forces me to re-examine a lot of other elements.
I realize that marketability isn't a thought I should launch out of the gate with, but I don't want to spend a lot of time writing something that no one will touch with a ten foot pole. I don't think this is an unreasonable concern. ;-)