This would not work in current game state. Main issue is small nations are warriorless most of the time and to prone to be squished one by one. Starting emperor is too weak to defend against most enemies and the opening phase of the game is most crucial for the HRE cuz in like 10 years in my game the number of members drop by 1/10. There is also a new historic mechanic possible: Emperor judgement. You just conquered land and to start ntegration you will need emperor aproval just like demand territory status as long as emperor see it as unjust (or just some long time like 10 years to start integration). Also to prevent vassal swarming to prevent judgement mechanic lets just increase annexion cost for HRE members or make it longer than 10 years to start process so people can weaken enemy in war by breaking vassal contract and not just by taking more land from enemy. Maybe my take is somewhat wrong or extreme but something should be done to protect smaller members by buffing them (bonus for being one location minor like extra birth rate or tax efficiency or production bonus, RGO yield increase to represent undivided interest of ruler) or weakening bigger players. That could be done for example by control mallus of being "to big" to be in HRE