I really like it, it's nice and easy on the ears. I know it was improvised so i don't think there is much point in me commenting much on the song structure. To keep it short, think you need to add a few repeated riffs and always have a structure in mind. Whether rondo, modern rock form (don't remember the name of this, but it's pretty much verse, chorus, verse, chorus, bridge, chorus). There's a list of different forms
here.I may be able to help you improvise. My guitar teacher had told me about this exercise "rip a riff a day." It would apply to piano as well i suppose. Try to learn a few bars of solos or improvisation a day by ear. Not only will it improve your ear, it'll subconsciously build a bank of short riffs in your mind which with practice you can call upon. On top of that, if you haven't already learn all about the modes and compatible scales, you should do so. Then you can mess around with some original riffs to add to your bank.
It probably sounds like you're just memorising other peoples improvisations, but it's not like that. It helps to visualise what you want to play. With enough practice any music in your mind can be instantly translated to what you play.
I'm still learning all this, but it has helped me a lot so far. You should give it a try.