Worst idea in the history of music gaming, but...
There are some critical flaws to this game that probably rank it among the worst rhythm games ever made.
The execution is a bit lacking. There is no way to scroll a few pages into a search or one of the existing categories, making most of the files made lost in cyberspace, probably never played again, or at least for a long, long time. Timing windows could also be more accurate, as the frame rate easily messes up even basic rhythms. On top of that, there is no way to see the file you recorded, and no way to edit it if you make a mistake. If you accidentally upload a file that is less than what you would like, tough luck, it's up there forever. Then when you actually play the file, the timing of the entire song is shifted, making every single note you hit offbeat. Click mode is also a very dumb idea, as there is no depth to its possibilities and the timing is extraordinarily difficult. Graphics are decent for the most part, but a lot of the color scheming in Frets make it very difficult to see the keys, especially once you go into the 8x multiplier thing.
The biggest mistake/flaw, however, is the concept. I understand that you want to create a vast rhythm game for everyone to be able to enjoy and make something. That is great, and I can totally support that. However, this game is probably counter-productive. Since players can only make files by recording, you can imagine not only the fact that it is near impossible not only to improvise a well-structured chart, but also to execute it without mistake. Given the frame rate limitations, timing issues, and poor design of the game itself, you can bet only 1 out of 1098257 files are barely even decent enough to be playable.
Despite all of these critical flaws, I do acknowledge that much work and effort went into this, and upon first glance it is quite interesting and did hold my attention for a while. In summary, my concern is that this game, while does have potential, will never be anything more than something a person would touch for a few seconds and then leave, creating a bad impression of rhythm gaming as a whole and ultimately rendering your game trash.
Rhythm simulators are not easy to create, as most people would think. There are many more factors to think about compared to genres such as platforming. Although creatively not difficult to produce, execution must be dead-accurate and bug-proof, or the game is useless and unplayable.
First, you need to gain a deeper understanding of the genre (and I mean a level of understanding above that of the average person), and then I would recommend making a sequel or redesigning this game to fit the needs. A rhythm simulator's goal is to emulate the sounds and playing of a song through gameplay. (I could explain in more detail if you wish. Message me if you're interested.) Yours is going in the right direction, but doesn't get the right result. I'm typing all of this because I see that your overall concept is a great idea, but until these major issues are resolved, this game will never reach its full potential.
I am sure this is one of the most helpful reviews you have ever gotten so far. I do want to see this go somewhere, perhaps even extend the project elsewhere, such as an independent website. If you have any questions feel free to message me here on Newgrounds. I hope I have been of help.
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