Add a first-person view to SpectatorPlus - so that the view is locked onto the player being spectated when they are punched, until the spectator presses CROUCH, which releases them back into normal spectating mode.

Enable first-person mode: Punch a player Disable first-person mode: Press CROUCH

Optional: There should be a book added to the spectator's inventory to allow them to view the player's stats (e.g. health) and their inventory and armour.

We can wait for the 1.8 for that, and use an API to enable the first-person view without the gamemode spectator (just like the fly, part of the creative gamemode, in every gamemode), if such an API is developed.

Optional: There should be a book added to the spectator's inventory to allow them to view the player's stats (e.g. health) and their inventory and armour.

We can sync the observed and the spectator inventories if the player is in a first-person view.
Warning, this said: possible performance problem.

Implemented in #40 (no-clip mode in the spectators' tools).

We can add a tool to see the inventory of the player watched (if there is an API for that).
» Edit: there isn't an API for that, as for now. I'll close this, feel free to reopen if needed (new API, new ideas, etc.).

I've been developing a plugin that does just this. I don't mind sharing the source, so long as you give credit. The plugin allows multiple people to spectate a person in first person .. all you have to do is create an instance of a runnable in a method I defined, and declare the update frequency. It tracks and syncs yaw,pitch,x,y,z,inventoryopen,etc ..