How about More Advanced Material for Polish?
Reached the max material for English-to-Polish and Polish-to-English a year or two ago, but still have a lot to learn to speak Polish fluently.
Wouldn't mind if there were some new material in the tree.
Another idea would be to provide some new guidance in the tree as to which items I need to work on most. All my lessons are at gold level for both courses. Don't feel like resetting and trudging through the basics for hours.
Try Clozemaster. Its what many people call duolingo after duolingo. If you have mastered duolingo you should give it a try. Don't worry about starting over as this website is not for beginners. They'll throw you into the water with whole sentences from the beginning. Duolingo can't teach you everything and clozemaster is the next step to becoming fluent.