I am going to go out on a limb and say the API from Mojang will never come. Jeb will keep pumping out updates, seeing ideas in mods and implement them into vanilla, and tweaking the code just enough to make things a lot harder in the end. I hope I am wrong but it is a gut feeling I have,
Keep in mind some people are bat feces crazy about being 100% vanilla.
Getting something like horses that's been in the modding community almost since 2011 for vanilla is like getting a Christmas present to them.
To be honest, Mojang takes the best of mods and adds them into vanilla, not mods that add random crap, like electricity or
cannons that sucks up the essences of animals so you can shoot them out in the form of effing pokeballs.
If you've noticed, the stuff that they add (comparators for example, just a single block but allows for construction of things that can replicate functions of blocks from mods and also allows computers) is extremely open-ended. Adding a block that sends out a redstone signal when it gets a redstone signal, so incredibly basic that it's funny?
LET'S GIVE IT 20 MORE USES FOR NO APPARENT REASON, JUST 'CAUSE WE CAN.
It's generally more fun to think of things with vanilla than it is with mods, since mods usually implement things that people couldn't be bothered to make in vanilla themselves so they just recreate it in a single block and give it a crafting recipe. Mods can't get away by adding fun things, like flying pigs, unless it's open ended. Very few mods have actually added open-ended things, one of the few being RP2 with it's frames. The redstone logic added by mods like IC2 and BC can be used to automate really complex and interesting builds, aswell.
EDIT: Spaced out because I like my posts that way and it looks like a wall of text otherwise.