My personal view: the next big improvement should come from tackling emotional self-management.
This is derived mostly from my own experience of ~15 years of struggling with personal productivity (and going through every trick in every popular book), as well as observing others close to me. In recent years I came to the conclusion that it's all about emotional state.
The planner, the TODO list, the Pomodoro timer, the Inbox & Someday/Maybe folders, the bullet journal - those are all complexity management tools. They address the problem of having too many things taking your attention, making you unable to focus. They don't address the problem of not being able to focus at all, not being able to open your TODO list, not being able to work on a task for more than 30 seconds before either feeling distressed, or beginning to question your life priorities. Conversely, when you're feeling really good and pumped about the thing you're doing, you don't need a complicated system to keep you focused - a notebook and some pens, or an open text file, is enough to keep the complexity manageable, and you'll figure out an effective enough system on the fly.
I've only begun exploring the space of emotional self-management, and so far, I haven't found any neat hacks or effective methods. A big problem is that the brain adapts to attempts at cheating it - like e.g. there was a time where I could control my emotional states through the choice of music, but it stopped working after I started exploiting it to make myself work on things. Similarly, all attempts at self-gamifying fail because I know I'm just manipulating myself.
But if anyone can crack that problem - effective emotional self-management - I'll happily shower them with all the money I can spare.