"I want to do it more often."
Translation:Jag vill göra det oftare.
Then you’re referring to a specific thing with an en-gender. Here you rather refer to an action or some sort of generic experience, and then you use a generic det.
Ah, ok, but in that case 'den' may be grammatically correct depending on the context?
eg, That pizza was good. I want to make it more often.
In that case you have to refer to something specific, you can’t be making the same pizza but perhaps if we’re talking about a specific type of something. Like a type of soup. You could say: Den här soppan var god. Jag vill göra den igen. Then it works, because you’re referring back to a specific type of soup. But as soon as you’re speaking generically, you have to use det. See Blehg’s post here.