The modding potential is fucking extraordinary. I think I might design a special Elective law for my mods. Now you can have electors favor whatever you want them to favor, so you really can make custom elective governments in whatever mold you desire (for example, if you were modding magic, you could make a Magicratic system which favors the most powerful).
I am so happy the HRE is finally its peculiar elective type. Personally, I like seeing a weaker HRE, and a solid elective session seems like it will inject a needed bit of chaos into the Unholy German Anarchy. And the historical electors preference is neat too, and the extra power that theocrats have under papal investiture is especially welcome as it'll give an actual reason to have an Investiture controversy. (Also, this was last week, but a vassal pope being much less of a yesman and the Pope in general being more demanding is very nice, especially for a player who wants to play Orthodox but can't justify it when Catholic was just so OP).
I am so happy that Elective is reworked the way it is. It was so frustrating that you couldn't affect electors, that you couldn't use favors to control them, and you just didn't know why things ended up the way they are. And now all of that is fixed! Hallelujah! Eldership also seem like an interesting mechanic for modding.
And I am so incredibly pleased about pretty much EVERYTHING that the Byzantines just got. Unique government with no penalties for holdings or different governments? Special backstabbing-based succession law? Royal court? I mean, dear god, I'm almost giddy. I haven't done a proper Byzantine campaign since my first one, and I liked it so much I was inspired to make not one, but two mods. But now, hehe, I might really have to do it again. Because WOW.