1. Totally rework a broken MAJOR current feature: World Regeneration
I think what you must divide world core (rocks, soils, sands) and decoration instances (prefabs, trees, boulders and etc.). The world core regenerate like now block by block. But at the same time, instances must annihilate block by block if broken by player, and after full clearing here (not exactly in this place, maybe near) will appear another random instance of same decoration. Trees and prefabs will change by time.
2. Tweak a broken MINOR current feature:
More interesting skill trees. That ones was described early are not satisfactory. I already told about this here.
3. Essential MAJOR feature before 1.0: Plant farming
I want to do a little farm of grass / flowers / trees where I want. More of consumable fruits and, maybe, a hunger mechanic.
4. Add a MINOR feature in next update:
With the hunger mechanic we need small health regeneration.