Out of curiosity, I wonder if newsletter diversity would be improved if you made suggestions by clustering the newsletters by content (similar to your other comment about seeding the network), but picking from the newsletters furthest from any cluster
Yes, I would like to do that--combine it with the rating prediction, so e.g. if two newsletters both have a predicted rating of 4 stars, prefer the one with content that's most dissimilar to the users' positively rated newsletters so far.