I think number 3 is the best, but 2 would probably sell the most. I'll try and explain my reasoning:
1. This one is like trying to teach creativity, it's tricky and might leave people feeling like nothing they do is creative if they don't think the same as the course. The advice for this would probably be to just play games you like and try out replicating mechanics from them and mixing stuff together until you get something you're interested in playing.
2. This one will probably sell well, because people know blogs are generally free and want to build a "huge audience!" out of their excitement. However, it's hard to guarantee that someone won't struggle with writers block because that's again promising to teach creativity, so maybe instead just give an outline of previous successful blogs that led to games getting noticed and sold recently.
3. This one might be the most useful, especially if it covers things to avoid like spam bots and general spamming of your game and #hashtags.
Maybe combine all three into one single course to make a decent pdf worth of content, and sometimes knowing what not to do is more useful than a formula that tries to tell everyone to do the same thing.
With the right research/case studies based on existing games that both were big hits and massive flops, you could make a really helpful guide/course!