I am currently working on a game of Ninja Fighting. I would be very happy if you play it and give me feedback later.
In the feedback there are some important things that need to be answered:
My game:
Enjoy! 
Not to sound aggressive, offensive or anything, but the game overall is quite bland. For a PVP map with a ninja theme, the map is very small. Moreover, I think the map needs more uneven plains. It’s too flat, which makes it easy to see everyone else, which isn’t a good thing for this kind of theme.
For and icon thumbnail, its quite bland and there is nothing special to it.
Moreover there seems to be a total of 5 unique free models (including the clothing individually), which isn’t advised since they take away the unique element of the game in total, as well as them sometimes containing viruses.
In terms of the genre, I advise to add something of your own element to the game. There aren’t a lot of successful ninja games, but the ones that do become successful have something that differentiates it from others. This applies to every game in general, not even just in Roblox. For example: elemental powers, exp levels, unique animations etc.
But I know that everyone can become a hit on Roblox if they are willing to sacrifice time and resources for it, you can become as big as any other developer if you work for it.