I know Seth told multiplayer wont be here any time soon on steam community, but there could be like local network co-operative mode. It wouldnt need servers so that would cost less and make players happy. Just my opinion
Just forgot to add that if you complete all difficulties you could get creative mode.
You could summon/give items, blocks, furniture, monsters you get the point

Have abillity to make yourself invincible, give yourself effects and modify the power of them (example: if you take speed boost you can choose how much percentage you get). Well this would just turn you into god.