What if MCreator implemented...

That would be very cool

And very illegal.

That's true in case that you do it directlly ,but in case that you use vanilla/forge methods and Java reflections it should bel fine, because you still do not edit or remove someone else's code

If you went into Minecraft code and made it so, ie: Grass dropped stone instead of dirt then that is illegal.

You do not need edit Minecraft code for it. There are simple ways that Mojang , MCP and Forge left for people. And these methods are as illegal as adding an item. (Because for adding item you must add your Item to method in Forge's class GameRegistry.class , and this method adds it to vanilla array. For replacing and making items de facto removed from game is is similiar. You have a class , that you put in method of Forge's class and it is used from there by vanilla code. (Btw , this one is for example MCreator's GUI overlays and global events))

acctualy there are mods that edit minecraft's code i have seen those mods before and there not illagal like changes the blocks to trinagles so ya your both wrong

This is a quote straight from Mojangs EULA and Terms of Agreement. The TOA, was updated in May.

If you've bought the Game, you may play around with it and modify it by adding modifications, tools, or plugins, which we will refer to collectively as "Mods." By "Mods," we mean something original that you or someone else created that doesn't contain a substantial part of our copyrightable code or content. When you combine your Mod with the Minecraft software, we will call that combination a "Modded Version" of the Game. We have the final say on what constitutes a Mod and what doesn't. You may not distribute any Modded Versions of our Game or software, and we’d appreciate it if you didn’t use Mods for griefing. Basically, Mods are okay to distribute; hacked versions or Modded Versions of the Game client or server software are not okay to distribute

Please note this line directly:
"You may not distribute any Modded Versions of our Game or software"

So, by Changing Minecrafts source code (making a square a triangle?) You are in breach of their contract and it is illegal. But, obviously nothing will be done about those mods as they are not causing problems. But, are still effectievly illegal as they violate the ToA.

TOA Link: https://account.mojang.com/documents/minecraft_eula

@#8 First off editing in game things with a mod is not illegal, however as the eula states editing the minecraft code (the encrypted one) is illegal. Using forge mods to edit what happens in game is by now means against the rules, unless the specifically find the mod to be bad for the overall minecraft experience, like griefing. A mod that changes in-game things but does not access encrypted files is ok. Your example of changing a cube to a triangle also doesn't make sense they added that as a feature of resource packs so once again it is fine without breaching and editing the encrypted source code. So mcreator could have edits to vanilla through forge code in their program, as long as the don't try and use the encrypted files because they are protected by copyright and are encrypted for a reason.

I think its not illegall at all. Mods can be removed and are optioal to install 

@xXMCXx and if you have Mojang's permission we cannot even talk. And Minecraft for XBOX by example, it got behaviour packs so you can modify the vanilla game. Did you seen any behaviour pack creator that got arrested for making it?

I have yet to say anything about "getting arrested" or any talk of punishments. Please do not put words in my mouth, I am capable of doing that myself. I am simply basing my statememts off of mojans copywright policy, I may be misinterpreting them, or not, but thats what I get from this. In the end, I do not believe Klemen will implement this future or make another program because MCreator alone takes up alot of his time already.

I am basing my statements off of their EULA, I may be wrong, but in the end this would require Klemens attention and I doubt he has time for another program of that magnitude, if he did, then there would ve less attention to MCreator. Most users prefer to make new ideas for mods, not edit existing vanilla items.

well my coundy dont know about