I understand what your saying that by going through proxies is not really spoofing your IP but being able to hide it. Editing the tcp stack seems to be the only way to really change an IP but seriously how many different ways are there to do that.

I like the idea of what you where talking about, predicting the TCP sequence numbers. That would obviously work, because by predicting it can't be 100% correct, it's going to be off and you won't have the original IP anymore.