I'm starting too late, so my goal is really going to be minimalistic - to add some actual playable content to the game! So far the game is just a tech demo of the engine I'm developing and while it helps me to test the mechanics, it's evidently not fun to play.
Just for the record, I'm starting with this (older picture, but not much has changed since):
So the goal is to add few more items, enemies, areas, effects, sounds etc. using the mechanics available at this moment.