How about a way to add lines of blocks? So if you have a wall 16 blocks wide it's a pain to press 16 times to repeat the same block. One way might be to click a start position then an end position and the editor would fill the line with the chosen block for you. A way to duplicate shapes. Often I have more than 1 complex structure and have to recreate each one by hand. You might have a way to select a range of blocks then copy and paste them. Have a row of the last used blocks at the top so you can easily access them. Otherwise you have to keep finding them again in the shapes. I often alternate between blocks, like the curved pipes and slopes.
There is an advanced Mekorama editor available online here (created by @Gepeto): https://mekostudio.com/3dbuilder/ It has some of the features you ask for. It can also import levels to work on them.
How about a way to replace blocks with another without having to delete the block first? A lot of time is spent removing blocks then adding a different one in the same space. Also how about being able to draw 9 x 9 squares in one go? Or stretch them to any size you need. These would be great for making floors!