Medieval MMO, players join historical kingdoms (they can't create kingdoms, only join predefined ones) and then elect one of fellow players to be their king.

Everyone can vote on any noble player. But the vote will count only if the player they voted on decided to be a throne pretendent.

The question is, should there be a list who is a throne pretendent or should it be secret?

The thing is the current king could try (realisticly speaking will always try) to make their competitors miserable/weak (like stripping from kingdom offices). Or outright call them traitors and punish everyone who is trying to associate with them. On the other hand, having a list who is the throne pretendent would make it easier to vote, also you could then know who got how many votes the last time.

Take into account the medieval setting, so it does not necessarily have to be a democratic election as we perceive it today Also the power of votes of players will differ greatly based on their profession, influence and status (the higher the noble title the more votes).
There will be also a parallel vassal system (a smaller group of players with one being the top senior), the seniors of each vassal group will get much more voting power (part of the votes of his vassal will be transfered to him). So there will be a clear minority of nobles who get an uproportionally high voting power.

Yes, throne pretendent could be almost skipped. It is only because some people wouldn't want to be the king (for political reasons, like if a group decided which one of them should be the king and the rest support him; also a king has several limits regular citizens don't have which makes this position not appealing to everyone).

Obviously candidates should be visible otherwise its not a voting but a guessing game that no one will care about, would you run for hours in a massive world and ask all your friends "are you in the elections this week?" ??? Its not fun.

You could always make the actual votes count invisible if you want people to compete more, no way to know if you're loosing or winning. As for the fact that king and his gang may want to ruin the opposition then I'd say let it be that way, if its an MMO about being king then thats great, if you're not strong enough to take on the current gang in power then why should you be a king? Would be a good idea to also add items or settings that help the strategy, like political immunity items which will protect you from anything during some time, so the opposition cant attack you... or a system where you can bribe other players to shift sides... you see the direction where I am going.

Maybe first about the "massive" multiplayer aspect There are 30 kingdoms and I expect 500 active players (of course I hope I'm terribly mistaken and there will be much more, but during the design stage I should stick to reality ). So, some smaller kingdoms can have like 5-10 citizens and 100 would be a really big one. So, a few people talking behind the scene to usurp the throne is not an unlikely scenario (especially if they are higher nobles and therefore have unproportional high number of votes).

Second, players are grouped in smaller vassal groups (5-12 vassals under 1 senior). Then if 3 seniors decide to take over the throne they could simply send private messages to their vassals on whom to vote. The vassal would listen to them in general since if your senior takes the throne then it means he will first give the most prominent kingdom offices to his vassals to assure continued loyalty. This way even 30 players could secretly decide on whom to vote without using any public means of communications.
And if the usurp fails... Well, no one will know except those who wanted to usurp the throne

Any other thoughts on this? Could be just how you feel it should work.