I was thinking about inactive players. Maybe indeed deleting after a month is too harsh...

Maybe like this?
- after 14 days of inactivity the player gets easily visible skull icon, also if that player is a ruler he can be "forced to abdicate" by any citizen ragardless of influence.
- after 30 days of inactivity the player disappears from the ranking list (but still appears on the kingdom list and city list), also his workshops does not count in the global game statistics.
- after, I don't know maybe 3 months, the player's character is deleted (the account stays, when he/she logins next time a fresh one will be created).

You could make it graded. Players who only played a few hours and then went inactive could be removed fairly quickly. Those who have advanced more should have a much longer time.
This would help against those who try it out and find out they don't want to play it. In Tribalwars, the large majority go inactive (especially on the rim. 9/10 is not uncommon), and it really makes the game feel very deserted.

I encountered this non terrible but annoying problem with inactive players and I wonder how to solve it.

There are various degrees of inactiveness:
- active today (* symbol next to player name)
- active in last 3 days (+ symbol)
- active in last 7 days (sleeping symbol)
- active in last 30 days (dead, skull symbol)
- active in last 3-6 months (hibernating, not appearing on most lists, player partially hidden)
- above this thereshold the player is deleted

Now, I need to display a number, how many people live in a city X Which one should I use? Generally, I was using "active/inactive" (two numbers) but it has two problems, first some players confuse it with limit of citiznes in that city, second... which "inacitve" should I use?
What's worse, I also display the list of city residents (with offices) which include everyone above hibernating, this is kind of needed to see who holds the office.
It's a big mess right now...

I don't know if it helps, I played a game similar to the concept but it was set in modern age, I think it was called Coloniam. It has implemented a voting and political party system and you vote for the people that you want for the president.

A bit too democratic for the medieval theme I'm using the system where high influence players get much more votes than the rest (weighted votes). As for political parties, I kind of have something of similar function, which is vassal system (usually your vassals will vote as you ask them, because if you are deciding who the ruler is then you have a higher persuasion power to make the ruler to grant lucrative offices/privileges to your vassals; well, maybe it's not exactly political party system, more like family bonds system).