An additional tier ! Mostly for modding purposes !
I do believe this will make things easier to describe some situations that are currently limited by current tiers number. This would especially help in balancing kingdoms.
For example kingdom of France is a big one and has similar power to the HRE. But the emperor is able to have vassal kingdoms, whereas the French king can't. Actually they were considered as peers and a level above other kings.
I would find interesting to consider France as a 4th-tier title, composed out of 3rd-tier titles that historically were Neustria, Aquitania and Burgundy (sometimes called "regna"), each one being composed out of 2nd-tier duchies. It is not important whether those French 4th-tier title would be called a "kingdom" or not, since this is localizable.
Similarly Germany would be the sum of the 3rd-tier "stem-duchies".
In my opinion those regna and stem-duchies are of same tier as less wide kingdoms such as the spanish or celtic ones.
The 5th-tier would be reserved to empires. The HRE could be one of these, as well as the Roman Empire or the Carolingian one. I would suggest the HRE to be a titular title and the Carolingian Empire (or Western Roman Empire) to be the de jure one, but it's only my opinion ! This would help big blobs to be more manageable (if needed !)
What are your opinions about this ?