A LOT of this is WIP at the moment, so I'll only focus on the "Player companions" mechanics at the moment

Feature 1: "Tardis Team" Party system mechanic

As seen in: Most Doctor Who Episodes

What is the "Tardis Team": The "Tardis Team" is a term used to refer to the current Doctor and who he is traveling with during a specified time frame. For instance, the Tardis Team of Moonbase included the Second Doctor, Ben, Polly, and Jamie. The War Games Tardis Team was comprised of the Second Doctor, Jamie, and Zoe.

• More details can be found here http://tardis.wikia.com/wiki/TARDIS_team

What makes the "Tardis Team" Different from Companions?: Companions are always companions, but the Tardis Team is always changing. It usually consists of 2-3 members, but has amounted to 4 a few times

Command: /tardisTeam <add/remove/clear>

Aesthetics: Your Tardis team's Members will show up in a kind of "Party status GUI" on the upper right hand corner of your screen as you play.

• Kind of like in Starbound, except that the data for the individual team members will be lain out a bit more like the Damage Indicators GUI.
• The Display will show...
  • Leader (name will be highlighted)
  • Status of Team member (will be displayed with an icon)
  • Currently held item (or last used weapon)
  • Health (Represented by a fraction of "current amount" over "total amount")
  • Name (or nickname)
  • A little message below the name (messages will vary depending on the status of the player)
    • Possible messages
      • Getting a bit hungry....
        • Player's hunger bar is very low
      • All well
        • Displays when player is perfectly
      • DANGER
        • Displays when player's health is dangerously low
      • Low Health... 
      • <HELMET/CHESTPLATE/LEGGINGS/BOOTS> BREAK!
        • Displays for a second when a piece of Armor is broken

Keybind:

• Undecided Key 1: Toggles GUI (Displayed, semitransparent, invisible_
• Undecided Key 2: Pulls up a menu detailing a bit more info (ie: Defense points)

Behavior of Mechanic in Boss fights:

• In a boss fight, the Boss will only Target players of your Tardis Team
• The boss will not be affected by:
  • TNT tagged as having been set off by players not in your Tardis Team
  • direct attacks by players not in your Tardis Team
  • If the team can, projectiles fired from dispensers initiated by a redstone signal not in putted by one of your team mates
• If enough time passes without the currently engaged Tardis team returning, the boss will reset its health and will instead engage the next Tardis Team which approaches

Limits on Tardis Teams:

• A Tardis Team has a cap Maximum amount of players or NPCs, so pick your teammates strategically

To be added later:

• More details Party mechanics in a boss fight
• Details regarding Companion Npcs in a Tardis Team

Details have been added regarding Tardis Teams in boss fights.

I plan on working out some mechanics regarding companion NPCS (like the ones suggested in this thread).

Likely this will include Tardis Bedrooms, player built houses, and maybe some respawn mechanics for some companions