The problem with what you have suggested is that the server would be 24/7 and therefore you could go on and it was easy but u come on 24 hours later and the gameplay was impossibly difficult because the server had been up for a while without you on it. This would, unless you were VERY VERY good probably frustrate most people who were on it and they may quit after the possibility of dying over and over. Also new players would stand no chance as if the server had been up for say a month they would join, play for a few minutes and then die. Its a good idea and i like it but after some thinking it wouldnt actually work sorry.
Unless you could find some way to make the server do some sort of "local" difficulty progression which would allow the server to advance the difficulty in maybe a 500*500 area and that would be an automatic claim sort of thing.
Sorry about saying all this negative stuff as i do like invasion but i just dont think it could be made to work very easily
-Vcokltfre
Cya!