I think if a player quits and is replaced by a bot, they should not be
allowed to rejoin the game. I also think if a player is disconnected,
after a certain amount of time (maybe 20 minutes) they should not be
allowed to rejoin the game.
The reason why is exemplified perfectly in a game I played the other day: https://youtu.be/IOZOye3tpL8
There's a summary of the game in the video's description.
Players know that most other players will let a weak bot recover and get into positions they'd never let a human get into. They can take advantage of this by quitting or breaking their connection when they are weak so that the bot can take over. This is clearly cheating. If players weren't allowed to rejoin games they've quit, and/or weren't allowed to rejoin after 20 minutes of the bot playing for them, this type of cheating could be prevented.
Very annoying to have defeated all human players in a long, challenging and very interesting game, only to find that the last AI opponent, stronger than me in troop terms but eminently beatable in a normal human v AI endgame, suddenly comes back alive and beats me in the normal way by playing like a half-reasonable human in the endgame. There has to be some penalty / restriction on this. Effectively they're getting artificial assistance. Obviously simply banning returners would be unfair on players with unreliable connections. Options :
1. time limit for returners as suggested above
2. ranking points are allocated when the last human who stays human is defeated
3. returner is announced to the game, and it's announced that they get no ranking points. You can play them for fun if you like, after you've defeated the humans.
4. disconnected players can get troops based on territories / continents, allocated by the AI (so they can stay in the game for a while) but cannot attack and cannot get cards (to incentivise rapid return after genuine disconnections). When the human returns, rights to attack and play cards are restored. Thus if they're still disconnected when they're one of the last 2 in the endgame they can be beaten by attrition, and if the human returns when they're one of the last 2 in the endgame, they are likely at a disadvantage, and the last human player has to conduct the elimination of the 3rd-from-last player in the knowledge that they're taking a risk if they don't ensure they will then be stronger than the last opponent.