Add a sandbox zone for drafting blueprints

Factorio has one where you can easily prototype designs without needing to leave the current session. https://www.reddit.com/r/factorio/comments/115m7dp/what_is_this_mod_called_i_see_videos_of_people/

Why doesn't "planning mode" work for this?

I use COI Extended mod's cheat module for this. It works very well. Insta-build and magic storages allows me to test all the blueprints without hassle. Infinite electricity, maintenance, population, etc. is also convenient.