To be fair, from my perspective, what looks like the North to you certainly isn't if you really are a Northerner. The Midlands are distinct to everyone but you darned Southerners, I suspect.

Down with Wessex!
Huzzah! Mercian and proud of it

But as long as you promise not to call me 'southern' I'll be happy.
I suspect the Midlands and North could have remained far more influential even under the hegemony of Wessex, as long as the genocide that the Normans visited on lands north of the Trent (the Harrying of the North) can be avoided. But I love the idea, BG.
I think you would have to avoid the Norman invasion, but then with a different line of succession leading to the first King of England you could do away with the excuse they used for their invasion, which
might prevent them coming at all. If the Normans invade, their main interests are still in France, and the focus of power will still shift to the south-east.
Assuming no Normans then, I think we could end up with an England that focuses as much on the Baltic as on the Low Countries and northern France for trade, and one where the basic language is likely still an English we could understand (common English descended from Mercian English as most people could understand it. We'd just lose a huge heap of French words, the structure would stay essentially the same.)
The counties would be quite different as well, as Wessex did away with what had gone before and imposed their own scheme.
I wonder if industrialisation could happen any earlier, even in part, with the English power base being much closer to the coal and iron fields of the Midlands, Lancashire and Yorkshire, and also further inland from the sea?