A. I really like the ideas . However, instead of taking 2 different paths(product and topic), I think it would be better to converge them into just one thing like ‘Term of the day’ and send this as a daily mailer. Investopedia already has this feature and has made me go back to the site many a number of times from my gmail. I am pretty sure that there are enough terms(tools and other keywords) in Product Management field to engage users for quite some time .
B. Attention is another aspect that can be used for incentivising people. Create an in-app leaderboard based on the points that people earn through every activity on the portal. I see that there are badges but putting them together into leaderboards which is visible to top product managers (and possibly hire these folks) engages people to give back more IMO.
Example: Leaderboards for Top Editors, Top Moderators etc.
C. Gamifying the experience though weekly quizzes and peer challenges. The points earned through this can again be linked to leaderboards.