That's a tough one! I will be looking into multiplayer for the home island as a start - eg hanging out with your friends on your home island, being able to see the host's built creations as he modifies them in real time.
For the main game with the storm, it's getting incredibly hard to do multiplayer with the new features added incrementally at the current pace. Unless there's another developer working on the game with me, it's going to be challenging to adapt the current mechanics for multiplayer. I've some a high level analysis of what multiplayer needs to support and here is the list:
- Buildings + Resource production
- Shared inventory items and item counts
- Settler counts + settlers AI (idle vs working) + settlers movements and animations
- Settler professions, school learning progress and status
- Individual settler hunger values
- Pets, pet abilities
- Happiness, electricity values
- Time of day, storm timer
- Storm cloud (sync lightning strikes, destruction of buildings)
- Islands' randomized sequence (likely seeded)
- Island dialogs chosen
- Island outcomes (eg windmill, time pause mechanic)
- Story islands' progress (eg Faraday's clues)
- Special buildings like shields need to sync their times struck, and shatterdomes need to sync their HP value
- Hot air balloon, trader, trader interval, trading items, trading blueprints
- Cards? Building unlocks? World collectibles?
- Sickness mechanic (tundra biome)
- Expeditions, planes (sky biome)
- Structural integrity (underwater biome)
- Quark bonuses selected, quark generation, quark quantities
- Turret bullets, bullet damage, storm HP
- Character traits
- Research levels
- Character accessories and customization appearance