Since people are posting ideas here. Here's a few I thought of that are geared a little more toward retention of players/getting people to invest in learning more strategy:
1: Something describing the common skill plateaus people reach (going from buying random stuff, to big money, to engines, to finer considerations such as shuffle control, PPR, etc.) Something like this could be helpful to show the strategic depth of the game, and get people to see that it's a worthwhile investment with enjoyment at each level.
2: How to hook new players. (useful starting kingdoms, dos and don'ts, etc.)
3: Suggestions for if you start leaving your IRL Dominion friends in the dust.