Galleblære said:
Exacly what are the base rules for being elected Emperor? Could I as Poland, for example, conquer land inside the HRE, improve relations with all electors (which are?) and then be elected? Or are there some "hardcoded base" candidates that need to be eliminated before new potential emperors can enter the playing field?
Any Elector, whatever his religion (even sunni or buddhist or pagan - believe me, I tried

), and any european catholic (CRC too I suppose - that, I haven't checked), may be elected Holy Roman Emperor (besides, at that time, that's more about Holy Germanic Roman Emperor). The key is having better relations with the Electors (you can see them in the religion screen, by hovering the pointer above them).
You also have to know that :
- Habsburgs (Austria and Spain) have a bonus (reduced now, as said previously)
- and that some Electors get more votes (check it in the country.csv : it's the last numeric data.