It's actually a ton of fun to start at a baseline and put your own twist on it. There's nothing wrong with that! Just like there's nothing wrong with starting from near scratch! But for me, I love taking an underdeveloped idea in a work or in a genre and bringing it into focus without having to reinvent the wheel. I find it really satisfying. Where I often find building up an entire world of ideas into a single piece of work really exhausting, because that leaves me so little room to chase down the ideas that really catch my interest.