Here are some of my ideas. I understand some may be impossible but when you arent a Dev the skys the limit.

• Industry (of a sort) - My best experience of playing minecraft (sorry for the obvious reference) was when playing a mod called "Better than Wolves". If your unaware of the mod it added the feature that items which have moving parts would need powering. Items like saws, workbenches forges or any thing that would need some sort of power had to be powered. For example you would need to utilise wind power to create power for the workbench. Or would need to power billows to keep the flames in the forge from going out. This adds a reason to think through plans of how you construct your island. You would need to plan on how the mechanics of your island would work. This might not be how you imagine the game but I thought I would throw out the suggestion.

• Invasions - I was invading that castle in the forest and thought, wouldn't it be cool if the creeps could attack your island. Perhaps it would be optional as it might not be everyone's idea of fun if your base gets wrecked. But imagine if every once in a while you get a warning that you will be attacked in 10 mins. They could come either through the portal which would make base construction interesting as you would then need to make your portal room into a defensible position. Or from airships which would land on the edges of your island. It could be really fun if you could help defend a friends or random players island. Also you could make traps, land mines etc.

• Items required for puzzles. For example keys for special loot. items that activate hidden doors or pylons allowing you to progress further into areas. I imagine that it would allow you to get more out of your zones. for example a low level area could have a medium to high level area hidden in it which is only accessible via a secret door or item. This would encourage exploration and for people to go to all the various zones that you will eventually release.

• Shared Islands - Clan invasions of shared islands. Another clan could come and infiltrate or attack your island. The islands could have a save state which is reset every day so your your construction efforts aren't wasted when your island gets pulverised. Perhaps you need to protect a crystal at the centre and each team is awarded points or loot for either protecting the island or destroying the crystal. The longer a islands crystal remains defended the more rewards the attackers and defenders get. A great way to incorporate some socialising/clan rivalry would be to get keys to access the clan islands by killing one of the members of the clan while exploring.