I'll be the contrary voice here and say I find the current balance of empires is almost ideal, minus OP Fatimids which need to start with 75% decadence or something.
The HRE is very well modelled. The problem is people applying what they know of Renaissance-era HRE to 1066 and expect similar mechanics and results. The Middle Ages WRE was not at it's Carolingian heights but it was definitely not yet a patchwork of weak squabbling states without any central authority. Depending on whom ruled it, it's potential was pretty enormous. Under competent leadership, it should not be uncommon for it to defy history and end up centralized.
The HRE is very well modelled. The problem is people applying what they know of Renaissance-era HRE to 1066 and expect similar mechanics and results. The Middle Ages WRE was not at it's Carolingian heights but it was definitely not yet a patchwork of weak squabbling states without any central authority. Depending on whom ruled it, it's potential was pretty enormous. Under competent leadership, it should not be uncommon for it to defy history and end up centralized.