ASK IH: What should I learn next if I am already a Developer?

I don't believe acquiring more skills will increase your motivation, but I do think getting traction on any of your shipped ideas will. If I were you, I'd understand distribution and how you can use it to get traction for your product. There's nothing more motivating to a developer than traction, whether its revenue or user growth. My two cents.

marketing.

It wasn’t until I worked (as an engineer) within the sales/marketing teams at a medium-large (1500+) company that I learned where the money is at. There’s a lot of opportunity if you can solve companies’ sales/marketing problems. But like what was said, you can’t learn in a vacuum. Otherwise it’s just an educated guess.

I think those other skills are hard to learn in a vacuum. Like development, they're learned through practice and building lots of little things.

Instead of thinking "I will learn to market/design/sell and then do a successful side project", you would likely make more progress if you learn "on the job" by creating and selling a small product.

Creating and selling a small product would get your hands dirty with teaching (content marketing), design (the content/blog/videos/etc and the product), marketing (building & continually emailing a list of potential customers), and selling (writing the landing page, writing the email sequence to launch the product).

I followed a path like this (with lots of help from Amy Hoy's 30x500 class) and I can say I've learned a ton about the marketing/selling stuff from actually shipping a small product (an ebook).

You already know what to learn:

I have been struggling on getting ideas to work on mostly because of my lack of motivation on some of them

Learn how to not rely on motivation to get things done.