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Command block: executes a command when touched, optionally passing the player who touched it as an argument. The commands are normal chat commands, and you enter the text in the same way as signs when selecting the block.

If the text entered isn't a command, the message gets chatted by the game (e.g. "Game: Player1 just found the code!").

An optional debounce would be configurable in the world options, or on a block-by-block basis, so any single block can't be triggered more than once per x seconds. The block would turn red when inactive, then green when active again.

This would allow for much more elaborate worlds, such as automatically giving a user edit rights or god mode after beating a boss. The more chat commands introduced in the future, the more feature-rich this block becomes.

To reach its full potential, we would have to have the ability to execute more than one command per chat message/block (i.e. /command1 <args> /command2 <args>).

Some possible new chat commands and their uses:
/placeblock <x> <y> <id>
/removeblock <x> <y>
By stringing these commands together, you can create entire sections of your level that change. Perhaps a single-player minigame square with multiple levels, pre-built boss arenas toggled by the game owner, or a countdown animation at the start of a race. The existence of this command would likely result in some bots that can scan a section of a game world and return the chat command(s) to instantiate those blocks for copy/paste.

/wait <seconds>
Yields before executing the next command in the chat / block, useful for countdowns or making blocks appear in sequence

/alias=name <commands+args>
If you've used the Linux shell you most likely recognize this. You can save a string of commands as a single command with a given name, to easily use it later. For instance, saving a string of /placeblock commands to easily change your boss arena on the fly. So you'd use it like this:

"alias=placeArena /placeblock 5 5 7 /placeblock 6 5 7 ..." and so on

Set an alias to an empty string to delete the command. Also the command /listAliases which displays the first couple commands of each alias would be useful.

/killallcommands
Stops any commands currently yielding with /wait, useful if you've made a mistake