The Meaning of Life, the Universe, and Everything.
I use MC mainly as an education tool for my kid. While he thinks he is playing, I put things for him to learn, here and there.
I'm constantly shopping around to find some things that would help me achieve my goals, and sometimes it's not easy.
For example, something that I find very useful is a big editable sign. Vanilla doesn't have it, so first I need to search for mods with signs, and then try them, and then, sometimes after spending time I can't find what I need.
So, my idea for a MOD is a collection of tools that teachers (or parents like me) could find useful. I'm not thinking about Minecraft EDU, that's something else. I'm thinking something similar to a building MOD, where you have a hammer, and other tools to help you build structures, only the tools in this MOD help people teach whatever they want to teach (maths, science, languages...).
.-Letter/number blocks. We could do crosswords inside the game and other things like that.
.-BIG editable sign or blackboard.
.-We could have a locked door, and a blackboard next to it saying "3+2=". To open the door, you write 5 on the blackboard. (using command blocks to check the text on the blackboard?). Or 4 letter blocks "3+2=" and you have to put the block 5 at the end for the door to open.
I'm sure there are lots of people out there thinking "I wish I had this", but most of the mods are gaming related, so it's difficult to find what you need. Sometimes you find 2 things in different mods, but they are for different MC versions and can't use them together.
If anybody else is interested, add your ideas for useful teaching tools, and hopefully somebody will do it.
You may want to take a look at computercraft. It's not that hard to learn and can do many amazing things (including interactive screens as "blackboard", or just a screen to type in the correct number and trigger a redstone signal that opens a door, etc.