Ripped from the history books! An empire where the royal family ascends to godhood upon death.
Pick a kingdom or empire or city-state. When a member of the ruling family dies, he or she might be elevated to godhood by the current ruler (or a later ruler). Even poor, unpopular or cruel members can be added to the pantheon. All that is required is a proper funeral and statue in the family temple (or an individual temple). Until the empire falls, the deceased is a minor deity who can grant spells, plot and intrigue with an extended family and hope to influence his or her legacy. Treat them as minor gods, or have them attach to an existing god as a semi-avatar of a traditional god (Livia-Hera, for example).
Those not elevated become ghosts, haunting their old palaces and sometimes visiting the common folk. Those whose remains are shamed (their remains dumped in in the river, for example) become demons or specters who sow discord among the people.
Be sure to read some Roman history when drawing up your pantheon. Or watch I, Claudius. This idea would work well in an urban campaign.