Instead of 20% being completely random, could you instead present some newsletters that are completely opposite to what they prefer? For example, if someone actively reads conservative political commentary, adding in some random newsletters about cars or travel doesn't improve bias (correct me if I'm wrong). It would not be better to offer some percentage of liberal political commentary? And vice versa of course. My suggestion is imperfect because I'm leaving out the middle/moderate viewpoint and touting only extremes, but I'm ultimately trying to drive toward not diversity a of topics but diversity of viewpoints on interested topics.