Brunaburh
Gone Fishin'
The idea of Wales expanding, taking parts of Western Mercia (plus maybe Dumnonia) is very interesting. Could Wales even end up being the strongest kingdom in Southern Britain? Of course, first it must unite. Would it be easier to unite Wales than to unite Danelaw? Maybe one would end up with three or four kingdoms in OTL England?
I think it really depends how much of a mess England is and how talented the Welsh leaders are. You don't need unity to expand a small kingdom from a hilly heartland, to soft flat chaotic lowlands. Welsh kings could find it easier to push into neighbouring areas of England when the local Danes were weakened for some reason than to fight each other.
The sky is the limit, but you need to make sure the Welsh don't take so much territory so quickly that they stop being Welsh and start being just another set of Christian Danes. That's why I posited a period of consolidation of the new borders to create a clear division between the heptanarchy and superwales.