They are all pretty good starting points for pieces, the last one being my personal favourite as it seems more developed than the rest.

Something to try and improve would be to look at where you can move your chord sequences to as having a repeated loop gets pretty monotonous after a couple of minutes. Developing chord sequences makes songs really become interesting. Take a look into The Circle of Fifths in music theory as a beginning point on that.

Another small point that would really improve on your sound; in the last song you have a rather quick kick drum blast going off. What you could do is edit the velocities of each kick to make the blast a little more musical and less 'machine gun' like.

Just my two cents!

Keep it up! Cheers!