Sorry to shill my mod on the main forum, but the HRE being too easy to unite is a common complaint I see here, so I thought I'd advertise this a bit:
Link's in my sig.Basically, HRE reforms after the first one cost more IA. Some reforms require a certain ruler stat. They all require you not to be a regency. I;ve also reduced the tax bonus from gemeinerpfennig to 20% and I've increased the BB gained from the modifiers. I did tone the army tradition loss of landfriede down to 1 per year though, so it's not all nerfs. When a human is the Emp only german culture nations will ask to join (no AI nations ask to join in 4.1 currently). Oh, and only the states that voted against revoking privilages will go to war with you if it passes, if they voted for it there's only a 10% chance of them changing their minds at the last moment.