There are API's (=Shortcut and Help to install Minecraft Mods and Rescourcepacks) like Forge or Modloader...
that's pretty cool. But how about an API pre-installed in Minecraft? And Even better - and That's what I want to Talk about -
an API inside Minecraft Worlds...
...yeah... but what's now better about Mods inside a Minecraft World? To set it simple at first you can install Tekkit there and build a Atomic Power Plant in your Minecraft World ... And then you give to somebody else who Never edited minecraft by this Mod, but who still can use your powerplant.
To get to the Data I would imagine behind it: There could be a Folder in each generated Minecraft World Where you can Drag in the Mod like in Forge. And it would run the Mod only on the World Where it was installed. 

Yeah..ok...but what is it now good for :/ ?
Mapmakers could incloude Mods like Portal-Gun-Mod in their Maps, without needing the player to install it. And Mapmakers who know about Java Coding and Minecraft-Modmaking can create Mods, that are needed for the specific Map
(for example: Portal Map -> Portal Gun Mod; Slender Map -> Slender Mod; Halo Map ->Halo Mod, etc.)

Or even further, Someone could create 3D Models for his porposes. As example The RIVAL REBELS Mod

What do you think? Please tell it to me here or on My Twitter: @midiOnTweet

Thanks for Reading! Hope MOJANG Reads this :D