I like the idea of Persistent Quests being dependent upon game events. To take it further, perhaps some of them could also be dependent on world/yearly events! Some PQ's may become available during "special" time period. Thanksgiving week may provide a PQ relating to Native Americans and Settlers working together, etc. Other PQ's could occur randomly for a week or month; such as one relating to a major railroad being built. It might require iron rods, hammers, spikes, wood, or other items. With each completion, the player receives a standard reward; and once the player has completed the quest enough times, he/she receives a special novelty product (like a commemorative railroad spike, etc), in addition to an increased, final reward.
PQ's that take place during windows of time would add depth to the game's story and play value -- something for players to look forward to.