Shift a point given in WGS84 coordinates

I have a question regarding shifting a point in WGS84 coordinates.
I have a rectangle given by its top left and bottom right WGS84 coordinate and want to scale it.
My idea was to compute the diagonal, multiply it with a factor => Compute the shift in longitude, latitude coordinate.
How do I compute the resulting point from this?
Thanks for your answer,

You haven’t mentioned the main point: What is the size of the rectangle (shift)?

For small sizes of several Meters you can do the calculation linear in WGS units (best in degrees decimal).

For sizes above several km (depending on desired accuracy) you have to consider distortion by spherical effects.
One possible way: Translate to a length-preserving coordinate system, do the computing there and transform back to WGS.
But attention: After back transformation it will be no longer a rectangle. You cannot preserve length, area and angles at the same time in any projection of a spherical surface.

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