Is it possible to expand in the HRE as Aachen? For the first 40 years of my game I've just been trading and advancing tech. Still an OPM and see no ways for advancement. I can't get PUs due to my government and I can't really attack due to the HRE and it's many alliances.
You just have to pick your spots; remember, the Emperor won't intervene if you have a legitimate CB. Find some friends of your own. Playing within the HRE can be a bit of headache, especially for an OPM, but if you're patient, you might catch a neighbor off guard.