WorldEdit and other similar mods are a great thing, but they are mods(data pack / resource pack) and therefore require to be installed. So if a built in WorldEdit-like feature was implemented, this would massively impact Creative mode and possibly other gamemodes.
Now, I know there are features like /fill but they are too complicated and just not enjoyable to use with since you have to calculate it all in your head for the parameters, etc.
Getting access to the feature
The feature is an item which is used like a tool. Basically it would be shaped like a magic wand.
Obtaining the WorldEdit wand tool would be only possible via cheats / Creative mode or both. In other gamemodes cheats would need to be on, and the item would be obtained by a /give command.
Using the feature
After getting the wand tool, right clicking on a block would open up a small menu where you could select the command like /fill or a sphere making tool, and select a block, size and parameters but with images.
For commands like /fill a grid-like boundary that expands as you walk in one direction, allowing for easy sight if you are making a square successfully.
Little unknown tip: pressing F3+G will display chunk borders. (sorry it's out of context, it's not that related to the tool)
Left clicking on a block will create something like /pos 1, then walking to another block will create /pos 2 while using the /fill command or something similar.
Left clicking and right clicking at the same time or pressing another key you can open up a larger menu which allows you to edit the map at larger scales.
For example, it will display a map, which starts at your player's coordinates and displays chunk borders, and one possible ability is to change the biome of the chunk, and possibly "paint" biomes.
Now of course making massive edits to the map will lag your computer alot, but while that is problematic the good outweighs the bad.
You can also copy and rotate buildings, but this would make structure blocks useless. Do you have any ideas?
In the large menu you can switch tools from the "quick menu".
A great built in feature that would improve Minecraft massively, but there are some downsides such as lag when making massive map edits.
I agree that SOME sort of 'worldedit' should be included in the game - & I like the idea of a tool (or even 'tools) that can only be got in creative mode.
IMHO I think Mojang have 'tried' to include some editing 'tools' to help map makers (eg structure blocks & as you said 'fill/replace' commands) but currently these are too 'clunky' to use so the use of a tool to mark corners & a GUI to select the 'mode' (cut/paste/copy/move) of the tool would simplify things.
Not too sure about the 'open map' suggestion though - I prefer a 'highlight box' to show what's been selected
I mentioned 'tools' earlier because I'd like to add the suggestion that we have a couple of tools..
> 1 for selection with GUI for cut/copy/paste [maybe even 'move'] - using these commands would 'place' the selection into structure blocks for ease of repeatability so using the existing [maybe modified] Structure block GUI yet having coord/size auto-entered into the required boxes when using the tool
> 1 tool for 'paint/fill/replace' with a GUI that allows you to select the 'block' to use, the 'size' of paint brush & probably the block to replace for the replace function
The 'downside' you mention could be limited by sticking to the 35x35x35 selection area of structure blocks (or maybe even increasing to a 3x3x3chunk [48x48x48] which IMHO should be big enough for most map makers) & that the 'paint/fill/replace tool will only affect the area withing that selection