I agree with Gilles.You should add a lot of vines and other plants.It will make the swamp look realistic,and give it the effect of untouched wilderness.
Dont overexagarate though.
Trees would be preety nice if you spiced them up with vines hanging from the branches.This would make give make them look a bit more random,which is exactly what you should need.
Add some color variations too.Blue mushrooms,yellow flowers,whatever you feel would look good.The only color i see for now is green,it also makes it all a bit boring.Again,overexagarating with color variations may make it all crappy.