This isn't mentoring, this is mutual collaboration between peers. But it fits here better than the challenge area.
If you have an epic sci-fi or fantasy story/world that you want to make, I'll help you with it and you'll help me with mine! I think it'd be really fun to jump into another artist's imaginative world and add to it! In return, you'll jump into my world! If you don't think it'd be fun to explore another artists' world, read no further!
I want to share my art, concepts, and story with you so I can get feedback and I'll do the same for you. You will add to my world by drawing/writing your own characters, cities, story arcs, technologies/spells, etc. and I will do the same for your world/story! And even though they're our own worlds, this is not a boss to employee relationship. Maybe you'll use my additions to your world or maybe you won't, it doesn't matter. That's not what it's about. It's about inviting another artist in and letting them explore! If you happen to like an idea, hey, great! But if not, there's no reason to stop the each other from making it.
However, on the other hand, you will tell me how your world works and I will respect your rules because I want to. I want to be a part of your world and you should want to be a part of mine. That's important. There's no point in adding to someone's world if you don't respect it. Otherwise, it would only be a one-way collaboration where we only take and don't listen. I want this to be a mutual collaboration.
If this is at all interesting to you, let me know. Again, this is purely for fun! Just two artists bouncing their imagination off each other