There are a few things you can do - the lack of a subwoofer in the back door will limit you however. The front door and rear pillar speakers are a standard size (can't remember offhand but easily measured) and can be replaced with two way speakers. I have seen people fitting 6x9 speakers in the rear pillars, I've not done this as the subwoofer is pretty meaty when connected to a decent head unit.
You may have all the wiring for a sub box in the back door already - mine has a squarish white plug which supplies power to the amp and feeds the speakers. It mounts onto several threaded inserts in the door with long bolts. If you have the hardware and can lay hands on the parts, it shouldn't be too difficult to fit one.
Also worth popping an A pillar trim off and seeing if you have the tweeter wiring (a smallish squarish plug with two wires going to it, tucked behind the plain plastic trim). If you do, the trim panels with built-in tweeters from the top of the range models can be found on ebay for about £20 and are a simple plug and clip fit.
