Please explore Mecabricks a bit more and you will see that it is already existing.

• Display the select tool panel using the top button in the left hand side bar (or press W)
• Shortcut for the select box is B
• To inverse the selection press I

thank you,
still, would having a dragable 3d box be cool and usefull? (example) Taking an entire building (with many parts) and putting it somewhere else in a city

There is not really such thing as a 3D box. A 2D box does exactly the same job.

well, a 2d box could have objects in your way/view and would not be usefull when trying to (example) hollow out an inside of an building or taking a single tree out of a large forest.

Is there a way to rotate a group of parts from somewhere other than the center?

Yes, use the pivot point tool (P key). It allows to choose a point outside of the selected components.

sometimes I want to put a beam on a special brick but I cant line it up. Using small grid is very sensitive which makes it really hard to place some bricks.

@MSmaug could you share a use case. E.g with a screenshot to see how I would do it or if something could be improved on MB. Thanks

how do I post a screenshot? it won't allow me to.

Use imgur.com and copy and paste here the BBCode.
https://www.mecabricks.com/en/forum/topic/14