Another thought:
When a new emperor takes over, they get an automatic +25 prestige, and a five-year bonus to for example, morale, manpower. This highly encourages nations to try and become emperor (AI should be coded to understand this)
BUT, when a new emperor takes over, the most recent one (or even two) reform is immediately cancelled.
Result: More nations try and be emperor, and the more often the emperorship changes hands, the slower the reform process.
In my game I recently became Emperor for the first time ever. It took some planning and strategising (and asking lots of questions on the forum!), but it wasn't that hard, because I'm already a very blobby GBR with Spain in a PU. What I was surprised about was that when I became Emperor, I inherited the exact same HRE that already existed. Austria had passed three reforms already, and I just took that over. I was then able to immediately pass the fourth reform with the IA I generated by adding all my territory to the empire.
It was kind of like I let Austria do the hard work to pass three reforms, then I just stole their work from them - with not much more effort, maybe less, than it took them to pass those reforms.
I would be in favour of emperor change cancelling at least one reform, as I think each new emperor should have to do his own work. In the most extreme example, I could have waited until Austria passed five reforms, and then taken over and immediately passed Proclaim Erbkaisterum and made it impossible for anyone else to ever replace me - despite the fact that I'd have only been emperor for a month, and Austria had been for 140 years. (Impossible except for getting 100% WS against me, but that's a tough call that the AI is unlikely to ever achieve.)
The HRE rewards are great, so I think the work required should be large too. And I think there should be more encouragement for nations to take it over, in the form of bonuses on becoming emperor, but that should also result in increasing the work involved.
I like anything in the game that encourages central points of importance, over which many nations are strategising and fighting and there's a tug-of-war over power. The HRE could be more like this, I think, if the above - or similar - was changed.