There are plenty of ideas that Mojang has said that they don't plan to add, such as the ones on this official Previously Considered Suggestions List. Even so, are there any such ideas (or ideas generally viewed as "modded" by the community) that you think would fit in vanilla and should be added regardless?
I'd personally love to see more furniture blocks like tables & seats and disagree with the idea that they "limit creativity". Wooden tables, for example, could just have a scaffolding-like model that would fit right in with the game's aesthetic (here's an example I made), and they'd open up so many options for building. Alongside providing a simple option for tables, you could use them as stools, parts of pillars, or legs of other furniture. You could even use the top texture to decorate your floor.
You actually make stone (the smooth gray rock) by putting cobblestone in a furnace.