I'm against adding too many more powers, it just makes the map more cluttered, confusing, and more difficult to find enough players to start.
My suggestions are:
Expand Ferrera to include Man and maybe Mod.
Transfer Papal home SC of Bologna to Rav.
Have Pisa start with Pisa, Gro, and Elba.
Remove Siena.
Add Austria with Trento, Trieste and Udine.
Have Savoia start with Aosta and Cha instead.
Make Briancon and Nizza French starting SCs (this makes it possible at least for a while for the French and Savoia to co-exist).
I personally think that the argument that the smaller powers can simply counter their seeming disadvantage by banding together or with active diplomacy as a flawed one, since the larger powers are faced with less need to conduct active diplomacy, so the playing field is still uneven due to the differing priorities of the powers. And sometimes even the best diplomats, so long as their nearest neighbor is not too compromising can't do anything. For example Siena or France. Siena will have to play a guessing game with Firenze to survive the first year, and no amount of diplomacy can change this if Firenze is determined to quickly wipe off Siena and dominate its area (there is the possibility a determined Papal state intervention might force Firenze to return to protect his home SCs, but then again this will amount to a guessing game on the part of Firenze on whether to turn back or double down, with nothing to do with diplomacy). Meaning, France in most cases will end up with just one SC after the first year and will basically be walled in by a larger Savoia. Short of a determined Milanese intervention, France is dead.
Here I tried to create a scenario as balanced as possible while having some semblance to history (Pisa owning Gro is simply for balancing Firenze). It still requires polishing, and Naples and Turkey might become a bit stronger, but overall everyone has a realistic opportunity for a solo.
I've played several Rinascrimentos: once as Venice, drawn (given these changed conditions, since I assume the "own Rome" requirement has to do with Venice's inherent advantage, which no longer exists, I suggest we remove it, else it gives an advantage to duchies in the South-West), once as Savoia, where I made a genuine effort to try something new with France, allow him to move past me, but just a year into it I learned through a long a hard battle that there is no possibility for Savoia and France to ever get along with the current map (somehow I managed to be part of the draw, although my size throughout the entire game barely changed as a result of France and later Venice), and once as Ferrera, with predictable results. I was doing fine between Milan, Papal States and Venice, but when the Venice juggernaut got going I (along with everyone else around me at the same time) was squashed like an ant.