I do think most of your schemes are variations of existing schemes, Kradie. This doesn't mean it is plagiarism. I did not study about plagiarism, so I don't know technically what can be considered plagiarism. Maybe searching on the internet we can find a better answer than my opinion. My opinion is that plagiarism is when you copy and paste someone's idea saying it is your own work. Example of plagiarism: if I play a game with a different scheme someone did not release yet and then I simply extract the file and upload it on TUS as my authorship. For me plagiarism is usually bad intention of someone to receive ALL the "glory" from the hard work of another person.
Note that sometimes coincidences happen and this is normal, for example, after a research I discovered that the first Trick Race map was from 2001 by a brazilian player called Stratovarius, but if you search in WMDB, you will find many very old trick race maps with other names, from 2005 for example. I don't think people even knew the Stratospecial map, they only had the same idea on a different time (the scheme wasn't that popular). Sometimes the coincidences are way too close from each other and make us think if it was a case of plagiarism or not. For example, the Darts scheme was released when there were very,
very similar schemes such as "Drill Fly" and "Drill Flier". Why only under the name "Darts" the scheme got well known? I don't know. I believe SiD is a very creative person since he created many other original schemes, very interesting maps such as the one with a big condom with a worm inside it and he doesn't seem to be someone who would steal someone's work or idea. I believe Darts got famous due to his influence as a competitive player and his strong link to the community. Even though I believe the ones behind Drill Fly or Drill Flier weren't credited or mentioned enough all this time. In my opinion it is a good practice to say the schemes that inspired you for creating your scheme or if it is a variation, just make it clear that it is a variation or cite the author of the scheme or the scheme itself you modified.
Everybody inspires on each other ideas, this is normal, this is how things evolve. Darwin and Wallace had the same idea and published that idea together. If weren't they, other people would release the idea of Natural Selection with other name, but it could take a long time for this. I believe some works of art are more creative than others, usually few are very creative introducing a new way of thinking and the majority just release alternate version of something it is already consolidated.
I'm trying to reunite and classify many of the different schemes I find around the community (check
Scheme ideas page), but I really don't know if I'm doing this correctly, sometimes people don't agree with my choices, but I believe this is just like the classification of living beings, it is not easy and this is made by an agreement of many scientists. This is what I like on a wiki, it is not a simple website, everybody can contribute or change something they don't agree, until a consensus.