The ability to replace blocks without disconnecting sections.
Sometimes I get into a situation where I need to replace a block, but in order to do so it would tear a chunk off of the grid. A common occurrence of this involves merge blocks.
It would be really useful if you could somehow mark a block to be replaced with another block. It could be as simple as just holding a button that lets you place a block in the place of the block you are looking at, with some sort of transparent guide you can see inside the block to see the orientation of the block you are placing.
Once you have done this, grind it down to the lowest state (with some protection to make sure you don't grind it to nothing by accident, perhaps just don't let you unless you hold down grind for an extra couple of seconds) and then weld it back up into the new block you want.
For blocks that are larger than 1x1 you could mark each 1x1x1 spot inside it to be replaced before you grind it down.
In creative you could just press a button to replace blocks you have marked instantly.