[JoshCalbet]

To improve codebase is harder than building from scratch, as you have to get familiar with it first and understand what other developers have done with it. Now you have experience you have a feeling of how to make things ~right. But as other mentioned the way to get better is actually to build stuff. Take a project/idea you'd like to see alive and chunk it into small/workable pieces that on the first place make a set of "deliverables" that keeps you engaged in your progress (you probably didn't have to do this with previous codebase). Then start the action! I'd suggest to focus first on the list of "easy" deliverables so you can get started and get confident as you connect pieces together. In the process you'll see how the list of deliverables grows but also you have to know where to stop (as most people from Academia do not know), for this ask yourself what is the minimal set of features that your project need in order to work? once you have gone through this minimal set you build from there and you can even return to see what can be improved (just as you did with improving codebase).

To get a projects/ideas look at your own needs and think what you would like to be improved to make your life easier or make some tasks less annoying or ask other people, there are a lot of people with ideas around there but very few people willing to invest time to implement them