The problem I see with the ERE and vassal limit is:
-The non-feudalism of the ERE is represented mostly through free ducal revocation.
-The vassal limit will force you, in all probability, to hand out kingdoms instead of keeping things at ducal level.
-Kingdoms can't be revoked for free, so the ERE loses the one thing that reflected how they were not feudal.
1.They could add some new mechanics too, you know. Like titles returning to liege upon the holders death.
2.They could make it so that ducal titles won't count towards vassal limit in ERE(to simulate that the entire ERE was emperors personal demesne in CK II terms).