The plot seems fairly sound, it just needs more to it. Right now, it’s a basic concept, and what you likely need is just to flesh it out, to worldbuild, and line up all the details.
So, by the looks of it, you have a guy who, in order to graduate his school, must successfully complete four quests given to him by four queens. Now, you’re on the right path with figuring out the basic idea for the quests before you move on to the details. And, there’s a couple ways you can go about doing this, depending on how you want to do it. But, I also don’t know if you’ve started worldbuilding yet, so you might be ahead of me here.
One, you can figure out the quests before you worldbuild, which would mean you have to build the land according to the quests. So, for example, if one of the quests is to retrieve an artifact from an underwater ruin, then you’d need some sort of water (river, lake, ocean, etc) with a ruin in it. And, thus, you would have to put that in your map when you worldbuild, or the quest won’t make sense.
Or two, you worldbuild before you make the quests. So, you flesh out the map of your world, figuring out which queen rules what part of the land, and so forth. You can also build your characters more, so you can, for example, figure out how stingy one queen would be compared to another. Then, once you have, at least, a basic map planned out, you figure out the quests from there. So, for example, you look at your map and see there’s a big lake in one of the kingdoms, so that queen decides to send your MC to go fetch an artifact from a ruin underneath it. And, so forth.
And, however you do it, eventually your world will come alive to you, and you will have endless options to do what you want with the story. You can add however many obstacles you want to the quests (like an ancient sea serpent guarding the underwater ruin), and you also get the details of the characters, their personalities, and whether one person will get along with another.
In short, it looks to me that all you really need to do right now is just flesh it out a bit so that it solidifies the plot.
Also, add in little details about the academy, even if the guy isn’t there. What are they like? What’s the academy’s goal? And, why do they send folks out on numerous quests before they graduate them? Could it be that these quests are there so the student gets experience in the field, along with proving to the academy that they have the determination to see a quest through to the end? Questions like this will definitely help fill out your world, and will also explain the primary motive of your character. Namely, he wants to graduate, so he must complete these quests given to him by the school/queens.
I hope this helps, grin.
