Kill the Character
I usually check with the player about this ahead of time, but most players really don’t care about their characters after they have given them up in their mind. Thus I have used this option to move the plotline along. By having the big bad guy kill off the character, it adds more incentive for the rest of the party to continue to pursue the BBG with more diligence. It makes it all more personal. And it shows that I’m not afraid to kill off their characters. And it shows how dangerous the BBG is. All together, there is a lot of reason to kill off the character.
NPC the Character: Enemy
Similar to the above, Kill the Character, this about making the character an npc enemy. In one of my games one of my players was tired of being the cleric (this was back in 2E days). He was tired of being nothing but a heal battery for everyone else. So he opted to make another character…which left his cleric available for my use. At the time the player characters were embroiled in a long campaign against a necromancer with armies of undead. So one night, the enemy had his vampires take the cleric and turn him into a vampire servant. Seeing as how the cleric was already high level, he became a powerful vampire and was soon a chief lieutenant of the enemy’s forces. Again, the player characters now had further incentive to fight the enemy, with the added bonus of having to fight their old friend along the way.
NPC the Character: Adventure Catalyst
This is the more “typical” thing to do with the character. The character retires into some sort of advisory capacity. Maybe they start working for a lord (or become a lord). Maybe they set up shop utilizing all their accumulated skills with goods that are useful to the party, such as information broker (sage). Somehow the NPC finds themselves in a position where they can easily aid the party.
The point is, this NPC is now a source of information and an adventure lead. The party is apt to trust this character more so than other NPCs which makes for an excellent way to move the party in whatever direction you want them to go. If they missed some crucial bit of plot information, this NPC has a ready excuse of why they are willing to hand over this info for free.
Disappear the Character
The unexplained disappearance of a party member can be a used as powerful motivator, especially if the scene of the disappearance implies it was not intentional. The missing character can be used as bait for other plots as the party pursues the “recovery” of their party member. Example: The party is following a lead as to where their missing companion has gone and they stumble upon a cult of demon worshipers that work for the Big Bad Guy.
You may know the character is never coming back into the campaign - The players may know the character is never coming back into the campaign - The characters do not know the character is forever gone.
Note that these techniques can be used any time a character is no longer being played by the player. The obvious loss of player control is when the player moves away or quits the campaign. However, sometimes the player simply wants to reroll with a different class. The point is that, whatever the reason, having a playerless character is a great chance to shake things up.