I did (am doing) something like this. But my goal has been to: 1) show my progress over time and 2) make tutorials based on what I’ve learned. The key with tutorials from a relative beginner is to make sure I am teaching correct techniques and habits. That’s why my tutorials have been focused on basic stuff that other beginners might be struggling with.
Anyhow, I’ve had a lot of positive response from other beginners.
If you want to learn by challenges - there are a ton of challenges here on the forum. Or set yourself to do a tutorial off CG cookie every day for a week. Or come up with a category to model each week for the next month or so. For example:
week 1 - model a weapon
week 2 - model a vehicle
week 3 - model a cartoon character
week 4 - animate a bouncing ball
week 5 - model kitchen appliance
week 6 thought week 10 - add materials and lighting to your above models
We could go on and on like this
Don’t rely on others for challenges. Just set your own challenges.
An even better set of challenges would be to think of a scene, break it into the various pieces you would need, then concentrate on modeling one asset each week. That way you will keep working toward a larger goal.