Dr. Robotnik 1, 2 and 3 Mix Needs work on instruments, expression
You've got the tune of the originals down right, now you need to work on what instruments you're using and how you're using them. One doesn't pound on the piano with maximum volume all the time, hit the cymbal with the same intensity every time, you vary it with the tune to add another layer of expression to it, without which it'll always sound a bit flat and uninteresting.
Your choice of instruments will have a big effect on what you can do with it. That piano instrument will be difficult to use anywhere you don't want an absolutely pounding piano -- even if you quieten it it'll just sound like someone hammering a piano 2 miles away, never smooth.
You may need to add more instruments to get a better sound for different parts. (If you can. This sounds like it may be computer midi synthesis, which badly restricts your options.) A good start but there's a lot to improve.