a wget command example: /wget someFileHost.com/download/73284922 ~ ~ ~5 it will download a structure block nbt file from online and place it down it could also make updating command block creations and adventure maps possible
(wget meaning web get)
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Sure, it's one thing if the people with a command-block can fill an area with lava or something, and you definitely should only grant that power to moderators anyway. However, the addition of giving people the power to download random files off the internet onto the host computer seems like it'd be incredibly easy to abuse.
Now, you shouldn't give stupid people command blocks, but even server moderators can fall for stupid tricks online once-in-a-while, and if someone was smart enough to get excited server admins the ability to download any file that is presented as a "Minecraft Structure File", you're going to have a lot of malware spreading around Minecraft Server Host computers.
I'm sure it's possible to mostly secure this command from malicious use, but it'd take a lot of dev time and wouldn't really be worth it. It'd probably be safer to, as a client, be able to send structure files from your computer to the host computer rather than downloading from a file host server for one. (Comparatively safer, of course. Not "safe".)
Also, why does the command place the structure right after downloading it? Wouldn't you want to just download and save the file, then use structure blocks to place it?
I get offtopic a lot, be warned. I also often mention a Minecraft clone called Minetest. The clone is nowhere near playable with it's default game in my opinion, but it's at least the start of a good open-source voxel rendering engine, and maybe one day it might be decently playable.