What if in the succession crisis 1066, Malcolm had decided to invade England in support of Tostig Godwinson and Harald Hadrada and a Grufudd who hadn't been murdered had decided to annex all the lands west of the severn?
Well considering that William has plenty of troops and the means to supply them, the invasions couldn't have been timed more badly. If William loses, however, yet leaves the Anglo-Saxon army very tired, weak, and depleted, then Malcolm and Gruffydd can make great headway...though don't expect them to hold the conquered territory for long.