How about a group of Buddhist nuns who spent their lives preaching against violence have their temple surrounded by the walking dead and have to figure ways to stay alive against them. Don't think that one has been done ?
Months later a new manga about a group of Buddhist nuns who spent their lives preaching against violence have their temple surrounded by the walking dead and have to figure ways to stay alive against them appears in top manga magazine, let's say SQ Jump. A huge hit. Get's serialized, then anime proposal. Tons of different merchandise being sold all over Japan and overseas. Buddhist nuns figures in sexy poses, body pillows, cosplay attire - you name it.
And some lucky fellow is getting all the royalties all thanks to Uncle Frank tossing a brilliant manga concept. Sackcloth and ashes.
Anyway, if you're planning to draw a manga, just draw a manga. If you're going to be any kind of artist, whether it's drawing, painting, music, or writing, your early works are almost certain to be either terrible, or derivative, or both.
It's just part of the creative process to churn out a lot of stuff that you throw away, and what you don't throw away still needs to be revised to be presentable. Fortunately, your technical skills continue to improve even with works that you throw away.
Experienced authors/artists/musicians don't actually escape this entirely, it's just that they have enough experience that with just a quick rough or maybe even with just imagining the work carefully, they can discard more of what won't turn out well.
The publishing time pressure and lack of time to rough stuff out and rework it is one of the reasons so many manga serials turn to crap halfway through. Once the publishing deadlines start catching up to the furthest ahead the creators have planned for, you start to get a lot of things that ought to have been scrapped but there's no time to do that. Of course, you'll rarely hear any but the most famous and established mangaka admit that. If you're not indispensible to the industry you can't afford to criticize it even obliquely.