So I don't have any tutorials for you, but I did some research in this topic. I'm able to observe and draw good enough from observation, but from imagination it's totally different topic and I have big troubles with it.
Great resource to listen about imagination topic is Proko: &t=1s
This movie contain one important thing: you can draw 10 years from observation and still can't draw from imagination. It's just different.
So what you can do:
1. Draw thumbnails, small, general shapes, without much details.
3. Choose one random silhouette and try create something new, just try too draw what you see your mind eyes.
( Anyway it would be great feature to have class mode with silhouettes)
4. Try research something more about "How to design something" instead "How to draw something" (change "something" to e.g "creatures")
5. You can check books: "Sketching from imagination", great drawings and some advices about sketching.
Please share your advices too, I will appreciate it. Unfortunately I still can't draw from imagination. So not sure if my few above suggestions are good. It's just my research.
Maybe you can try something little bit different. For example change your topic. Try different exercises. Do warmup before drawing session. Do exercises like: 2 drawings reference photos and then draw same thing from memory.
Practice your foundation: Form, Gesture, Structure and Perspective.
