I think that Marketplace, Recruiting and Training are all a very natural fit for Hashnode. I also believe one of those will have the most potential for Hashnode monetization. Whether it works or not comes from the execution. It really depends on which form it is connected to Hashnode and how the platforms would work. One of those done right is probably also better than several done half-baked. Personally, Training and Courses would be great for me.
I'd not be happy with ads. The paid feature for devblog might also be interesting for Hashnode. And I like the merchandise idea. This seems like a why not.