How to rectify polygon shapes in QGIS?

There is a QGIS Tool called Oriented minimum bounding box. You can find it under Processing > QGIS geoalgorithms > Vector general tools. I'm using QGIS 2.18.

Maybe this comes close to what you want to achieve.

You can fix it with bounding box area of all polygons. The name of function is Polygon from layer extent.

Select your layer and mark the option calculate extent for each feature separately:

All original polygons are present in the same layer, not separately. QGIS will calculate the bounding box area for all entities:

QGIS Rocks! Hugs from Brazil!

Jorge Santos