When it come to writing music, there are many ways of approaching it.
In my case, over the years I have grown my musical taste so what I tend to do is write a piece in say a classical style but then I will also use that theme and write that piece in a synth version and maybe guitar version and so on.
The best part about this way of composing is, once you have done all the hard work of working out the first piece, the rest is very easy as you have the music so all you have to do is just re arrange it to suit the new style you want to write it in.
Also, doing different versions of a piece in different styles gives you new ideas for that piece too.
I also sometimes use a motif from one piece and put it in a new piece I am working on if it fits. I don’t see anything wrong with doing that as that piece is independent from the first piece. Also what that sometimes does is if you do that on enough pieces, it could give you a complete score with all the cues using that motif.
There is so much we can do with just one piece of music.
If you are an artist, you paint your picture and that is it, that picture is one moment in time but when you write a piece of music, it could be endless and take many routes.