I've made a fair chunk of contributions to the wiki that I am very happy with. The External Editor page is solid, if not particularly flashy. I've written a couple of tutorials of varying complexity. I've poked around in the scripts, I've edited maps (badly), I've edited events, and I've made my own sprites (by ruthlessly butchering existing ones). It's time to turn it up to 11.
I'm going to re-implement Pokemon Fire Red, from Pallet town to Pewter City. Every frame, every item, every trainer, every line of dialog. Implementing a portion of one of the core games is the best tutorial I could possibly conceive.