At least one major protagonist per expansion in my opinion.
Ideally someone who is fairly important rather than a throwaway character specifically created to be killed off in the same patch/expansion that they're introduced in.
It's a fair compromise - and allows for new characters to gradually be weaved into the story. After all, the cast is becoming very bloated and we've already seen some major characters reduced to only showing up as a brief cameo standing around at a meeting table with no actual dialogue.