I love the world and characters I've created for my story, but I dislike the plot. How can I proceed?

It may add an amusing undercurrent if the friend was actually not a jealous idiot, and only the primary protagonist knows this - the jealousy is a public front used in an attempt to determine who the protagonist's friends really are. His acts of evil in "helping" the bad guys always fail in such a way as the blame points to one of the evildoer's underlings, who promptly get wiped out by the evildoer.

Let your characters write the story for you. What is your MC's weakness? What could happen in your world that would force them to face that weakness head-on? What decisions would your MC make at key points? Were the decisions difficult to make?

Keep a loose vision of where your story must go, but let your characters breathe and the plot between those points will unfold as you write.

The plot does not affect the characters. The characters affect the plot.

Given I've written about a quarter of my story so far, which defines my characters and world and sets them up for their journey, what is the best way to proceed?

Ask your characters what happens next.

You know where your foreground characters are. You know them well enough to know how they'd react to certain situations. So: set up the characters who haven't turned up yet.

• The hero's dad: where is he? Why?
  If you want them to meet, set up a situation where that's likely to occur – perhaps put him somewhere where he can hear about the rag-tag team and decide to try to meet them, or where they end up trying to solve the same subplot. (If there's some kind of prophetic destiny thing going on, perhaps have multiple ways they might end up meeting up.)
• The former friend: how do they get from "best mates" to "trying to end the world"?
  If this is going on in the background while the protagonist's plot is going on – which is sounds like it is – make sure you can follow what the ex-friend was doing during the first quarter of the story.
• The strong, unknown enemy "arises"?
  Don't just have this enemy pop out of nowhere. Everyone starts somewhere, and the enemy is no exception. How did they start off? How did they attract the former friend's attention (or was it the other way around)? What were those interactions like – did they affect how they see each other? How did they interact with the heroes' wake (and how do the heroes interact with theirs)?

Once you know where everyone is and what they're doing, and know them well enough to predict how they'll behave, sketch out their stories. Don't go for grand narrative arcs; go for "what's this character thinking? What's their plan? What will they do next?". With any luck, your plot will go completely off the rails and start to get interesting.

I say "sketch out" because it doesn't have to be well-written; most of this will just be behind the scenes, unstated character history. And if you want your characters to make different decisions, or meet at exactly the right time, you're still allowed to go back and edit the entire history of the world, or nudge a couple of character decisions slightly (if it doesn't make them act uncharacteristically), or even tweak the characters themselves. You are telling a story, after all; all the world and characters exist solely for that story.

Your Big Bad has goals and values, just as much as your protagonists. Everybody has a price: everybody can be swayed from their plans if they're convinced that something else is better for them.¹ Everybody makes the best decisions they can, given the options they think of at the time and how good they think those options will be. If you write all your characters like this, even when the "camera" isn't focused on them, your plot might end up more likeable.

A downside of writing like this is that it can be hard to get your characters to actually do something you want to write about, without doing so much interesting stuff "off-camera" that all the conflict looks like a big "deus ex machina" / "deus ex diabolica" fight. I haven't figured out how not to do this, other than pushing the characters in appropriate directions (or setting up a rich, creative world and jumping between multiple viewpoints, which I personally don't have the skills to do well), so YMMV.

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¹: I don't mean "all your characters have to be selfish straw Vulcans"; by "better for them" I mean "better according to them" – this includes deals like "yourself to die, and the child to live".

I have been having the same problem, so the amount of help I can give is limited. I will say this, on a more constructive note. Originality means little, writing is 99% execution and 1% original thought. As some might sarcastically say, "copying from one source is plagiarism, copying from a whole bunch is inspiration". As an example, take a look at Star Wars. The original trilogy. A New Hope was deliberately written to be the most generic, schlocky thing imaginable (down to following Campbell's monomyth beat-by-beat), and people loved it because the execution was so great.

Same with Lord of the Rings. You'd think after 66 years of imitators somebody, somewhere would come up with a distilled version of the heroic fantasy that would make the original forgotten except for its historical novelty, sort of like what happened with Burroughs' Barsoom series and science-fiction. Except it's not, because Tolkien's execution was so dang good.

As Xavier, Renegade Angel would say, "it helps no one to be reductive". Any idea, when you boil it down to its most basic components, can be framed in a way that sounds trite and overdone. It's the bells and whistles, the twists, and the way you put your unique stamp on it that make the work "good". Even your run of the mill, "a hero joins a rag-tag bunch of misfits to destroy a dark lord" story can be good if its presentation is suitably unique.

What you need are two hooks, an environmental hook, and an emotional one...

Environmental - Why is your setting unique? What differentiates it from every other fantasy world out there. The magic system? Mistborn got a lot of attention (indeed, probably all of its initial attention) due to its unique magic system. Dorohedoro got a lot of attention because of its unique slummy setting. It's things like this that make your story stand out, and you can even make very basic plots seem fresh if done in a novel enough setting where audience expectations are not established, and anything can happen. Segue into your own unique take on a plot, and you have a recipe for success.

A good example of how this could be done badly is The Inheritance Cycle. The Inheritance Cycle had an interesting magic system, and if the plot were designed around that, it would have been an interesting story. But instead, we got Star Wars with dragons. The author couldn't find the hook.

Emotional - Why should I care about these characters? What makes them different from your cookie-cutter fantasy archetypes? What about their personalities and relationships with each other makes them unique? You say that the hero fights his best-friend-turned-evil-because-he-is-jealous-of-his-power, but how does that differ from Naruto and Sasuke, or any other number of similar characters? When someone can read your story and see your characters as unique characters with their own voice instead of "Plucky Hero #7208", you know you've done your job.

If you do want to revise your plot, rewrite it, so it highlights the good aspects of your story (world and characters). When I was designing my story, I put my magic system and characters first and then devised a plot that would let me showcase as many of those aspects as possible to the viewer like I was building a train tour through Jurassic Park. Play to your strengths.

At the center of every story that really is a story, there is a character who changes. If none of your characters change, you have no story, just a sequence of related events.

(For an example, compare the two films Cars and Cars 2. Two characters change in the first movie. Nobody changes in the second, and so the second film is forgettable.)

Look over these characters and find something about them that needs to change, or decide what path they tread to become the people that you like so much. Then start your tale at that earlier stage and run it to the completion.

In my opinion, you've already done a lot of the hard work. Creating the characters and well-constructed world-building leaves you open to follow endless avenues of plot. This is why fan-fiction is so popular because once you have the characters and the world, the plot can start writing itself.

That said, as @JonStonecash has said, finishing the first draft is so important. I have been in your position and immediately fell back on, 'I hate my plot. It's too similar to x y z. Let's start over!' and there's a lot to be said for finishing your draft. You may fall in love with ideas you have along the way and then wind up somewhere you never thought you could be, or you get to the end and realize you still don't like it. But I guarantee the journey will expose ideas and plot you otherwise wouldn't get.

First of all, there are only so many plots. It is unlikely that you will come up with a new heretofore unseen plot. It is not the newness of the plot but the telling of the tale that is important.

Second, I think that your priorities are askew. Your first priority should be to finish your first draft. Then, and only then, should you worry about the issues that you raise. My first question to that end is, how do you want the book to end? That is, what is the situation at the end of the book. Heroes valiant, evil deposed, that sort of thing. Write that last part of the book. Now, what has to happen before what you have written to get you to what point? Think of the penultimate situation and the events that would take the plot from that penultimate situation to the final situation. I would think that there have to be several different ways to get penultimate to final. Think of ten ways, twenty ways. Pick the top two or three that strike your fancy. Write up each one of them to see what works and what does not.

Third, do not think of your book as a linear path from start to finish. It will eventually have to evolve into that form to be published, but during the writing, think of the multiverse. Think of time travel. Think of things in your ending chapters that make demands of earlier material. Think of the book as a living garden; there is always something that can be pruned/potted/whatever in such a garden.