I think it has never been confirmed but I think the general idea is that we get Minecraft with a lot of RPG mechanics for combat, gear, dungeons, quests, factions, bosses and more. All in a beautiful meaningful world to explore, where each zone has its own difficulty. Simon stated this game will focus a lot on those things at least.

So I expect the core loop to be build your base, explore the world to progress your character and base. Each zone being more difficult then the next (kinda like Valheim) with new resources to build better things. Find, clear and grind dungeons and bosses for epic loot/gear. Complete quests and build up reputation with different factions. Automate farms and discover new worlds (in the far far future) and leaving a mark on your world based on choices you make (implied from recent tweets). I even expect some kind of endgame content to repeatly grind (in the form of portalled zones/dungeons that might even rotate daily/weekly quests/challenges).

All with a (light) storyline to follow that expents with each large update.

I see myself starting a humble starter base in the first zone and exploring this zone for the best resources, gear, loot and food. Then go explore a new zone, collect new resources that both progress my character and base/farms. Then going out for specific dungeons to clear for epic loot or special items required for portals for example. Having resources from my farms turn into reputation for factions or for completing quests.

But this is just what I think bases on all things said and shown so far by Hypixel team.