I've been a member since July 2004 but have mainly read posts here looking for technical help. Here's a short snippet of a demo using a variety of fairly inexpensive sample libraries. Any and all comments or advise are welcome. Here it is, take a listen.
I would just keep on working. Think of the piece as a diamond. You've knocked the first set of facets into it. Now, knock facets onto each of those facets.
I would suggest broadening the piece a bit, and getting some dynamic sweep into the overall shape. One way to work at this is to figure out where the emotional peak of the piece is, and build it out to its full glory. Then, working backwards, figure out how you're going to build to that place.
If you are using a sequencer, be sure to get into the piano roll view, and listen to each line horizontally. Do what it takes to make the best possible part by editing note tips and tails and velocities. Then use CC11 expression controller data to tweak the phrasing further. Make sure each part is being "played" as musically as a real player would approach it.
The only other thing I would suggest is that if your sample set is limited, you will want to write things that sound good on those samples. For instance, there was a trumpet line in there which is nice, but the samples are a bit hard-articulated for that part. If you don't have other trumpets, you might want to give that part to an instrument where you have samples which will express it more musically. Or, you may wish to keep the part in the trumpet, but alter it so that it is more characteristic to the articulation you're using.