Some questions to ask about could include:
How varied should biomes be per world - i.e. each world is one extended biome with 3 or 4 block types or each world is made up of lots of different biomes representing a varied and interesting alien world.
Do people like the existing trees - should leaves deteriorate after wood is removed? Should there be more varied sizes, shapes and branch distributions?
Combat: MMO style tab targeting and skill rotations vs spam click on the mosters - should the game be more action style in it’s combat like black desert online or minecraft with real time attacks and dodging 
or should it be like wow and gw :(.
How do players want to handle communication in game - traditional series of chat windows allowing players to talk to everyone or something more “immersive” like chat or emotes only being visible to players within a certain distace.
Racial abilities - what kinds of unique abilities do players want to be able to choose from - combat, building, crafting, gathering, utility related.
Moving around - besides portals, what options do players want for moving themselves and items around short, medium and long distances. Railways? Gliders? Movespeed boosting path blocks? Short distance intra-world portals? Item tubes? Cheaty global access to all items and blocks?
Item storage and sorting - do players want to manually sort their items betwixt chests, have their items automatically sorted by the game or do they need to build contraptions that facilititate sorting.
Utility blocks - what kinds of special ability blocks do players want to be able to craft? Move speed boosting blocks, bouncy blocks, slippery blocks, mob-spawn-blocking blocks, slowing blocks, moving-other blocks-around blocks, healing blocks, lighting blocks, noise making blocks, etc.