Depends on what info you want in the file name. Maybe something like
%artist% - %album% - %tracknumber% %title%
It is really up to you how you want the file to be named. Just don't include any / in the file naming string, as this marks a folder.
Downsides are obviously that you end up with lots of files in a single folder, which can be hard to navigate. Depending on the file system you could hit a limit or the performance suffers.
I seriously would just use a player which will also read songs from sub folders. All the players I have used so far on Android had no issue with that.