I know that other GMs must have solved this problem, but it is still perplexing me.
Since most of the "Campaign Traits" that have been seen for Curse of the Crimson Throne, Second Darkness and Legacy of Fire, deal with the first adventure (and with CotCT and SD the first encounter), how are replacement or added player character traits being handled?
In most of these cases, the new character will simply not have been present for the event that "forged" the party.
I'd say they're screwed ;-P
I don't think we'll get campaign traits for all adventures so latecomers can be introduced, so you'll have to do it yourself.
It makes kind of sense, because the longer you've played the AP, the more you made it your own.
There are three ways to handle this that I can think of:
1) The replacement PCs take one normal trait and one campaign trait. This likely will requiring 'reflavoring' the campaign trait, but the mechanical bonus would remain the same.
2) The replacement PCs take one normal trait and lose out on the other trait for not being original characters. This provides an incentive for players to get a character raised instead of just rerolling (since its cheaper.)
3) The replacement PCs just take two normal traits.
You can also mix and match: Using option 1 or 3 for characters killed in a way that would make raising them troublesome (death effects, disintegrated, eaten by a Bargeist, etc.), and option 2 for normal deaths and voluntary retirements.
Just remember that in many situations, especially in adventure paths, raising is just not an option.
You'll often be several weeks' travel away from someone who could, and would, raise your companion. And even then, you'll have to be able to afford it.
Not to mention that you don't always have a cleric in the party (and even if you do, there's a 1 in 4 chance on average that it's the cleric who needs the raise).
Beyond that, it can always be a character decisions not to get raised. Some characters don't want to defy nature like that.