Initial thoughts as I piece together a timeline
I think a better POD for ending Northumbrian dominance might have to be before the conquests of Lancashire and Chesire in 816 and the defeat of . As it is, the pagan Eanfrith of Bernicia has a pretty good chance of reconquering Deira, which Cadwallon had apparently shown more interest in razing than in ruling. Eanfrith would have a harder time than Oswald did, but Northumbria still has a chance. So it's crucial we get someone opposed to the pagan Bernicians in power in Deira right away, someone not strong enough to just conquer Bernicia, but strong enough that Bernicia can't conquer them or focus its energy northwards.
With Northumbria crippled, the big winner in the North is Dal Riada. The luckless Domnall Brech was king of Dal Riada at the time.
Wikipedia said:
Domnall suffered four defeats after he broke Dál Riata's alliance with the Cenél Conaill clan of the Uí Néill. In Ireland, Domnall and his ally Congal Cáech of the Dál nAraidi were defeated by Domnall mac Áedo of the Cenél Conaill, the High King of Ireland, at the Battle of Mag Rath (Moira, County Down) in 637. He also lost to the Picts in 635 and 638 and lastly to Eugein I of Alt Clut at Strathcarron in 642, where he was killed.
Besides the four named defeats, Dal Riada had also backed Oswald's invasion of Northumbria, and most of the troops present at Heavenfield were Dal Riadans. So Dal Riada may even be weaker at the beginning. However, Alt Clut and the other petty kingdoms of Cumbria and the Scottish Lowlands will probably not be a buffer between Northumbrians and Scots, and most probably will fall into the Dal Riadan orbit. Likewise Pictavia, which in OTL was a Northumbrian tributary from the late 7th century.
Another thought is that pagan Bernicia, left without allies, will merge somehow with the British kingdoms of the lowlands, creating a sort of proto-Strathclyde (which in OTL did not emerge until devestating Viking raids in the 870's)
In central Britain, we are assuming that the power vacuum in Deira is filled by someone hostile to Bernicia. Making a pretty big leap based on tenuous facts (the alliance between Cadwallon and Penda is shrouded in speculation, mystery, and outright propaganda), I can perhaps posit a Briton related to Cadwallon, with some claim to the area based on a connection to Elmet or another extinct British state, probably in some form of marriage alliance with Penda's house. This person probably will be a Christian, and his kingdom will be the first merger between Welsh and Mercians.
Wikipedia said:
One manuscript of the
Anglo-Saxon Chronicle says that, following the victory at Hatfield Chase, Cadwallon and Penda went on to ravage "the whole land" of the Northumbrians.
[27] Certainly Cadwallon continued the war, but the extent of Penda's further participation is uncertain. Bede says that the
pagans who had slain Edwin—presumably a reference to the Mercians under Penda, although conceivably it could be a derisive misnomer meant to refer to the Christian British—burned a church and town at
Campodonum,
[28] although the time at which this occurred is uncertain. Penda may have withdrawn from the war at some point before the defeat and death of Cadwallon at the
Battle of Heavenfield, about a year after Hatfield Chase, since he was not present at this battle; furthermore, Bede makes no mention of Penda's presence in the preceding siege and battle in which
Osric of Deira was defeated and killed. Penda's successful participation in the battle of Hatfield Chase may have elevated his status among the Mercians and enabled him to become king, and he may have withdrawn from the war prior to Heavenfield in order to secure or consolidate his position in Mercia.
In OTL, Penda's rule of Mercia, apparently, was not completely centralized, and Oswald was the biggest threat to that dominance. With Oswald out of the way, Cadwallon will be the biggest threat to Penda's power. In OTL, Penda allied with Powys against the Northumbrians, but whoever he winds up connected with, there will be conflict for control of Deira and Mercia. It is also claimed that his half-sister was married to Cadwallon, which certainly could have a big effect on the dynamic between the two.
In OTL, Penda fought against Oswald of Northumbria for much of his life, culminating in his victory at Maserfield in 642. After the Northumbrian threat was ended, Penda subjegated the West Saxons and contested the East Anglians for control of Middle Anglia. Let's assume for the sake of the scenario that Penda spends most of his life contesting for control of Deira and eventually Wales, which he will eventually wind up on top of, in dynastic alliance with various Welsh factions (most likely Powys) and his descendents Christianized.
With Penda looking towards Deira and Wales, who will control the south? I would guess a more powerful East Anglia dominating Sussex and Kent, with the southwestern kingdoms maybe allying to contest Mercian/Welsh dominance. One thing that should be noted is that if Mercia can stay pagan longer than it did in OTL, the East Angles and Saxons will probably be Christianized from the Continent.
This is very sketchy (especially south of the Humber), and I'd appreciate anyone who knows Britain at all (or has as much time to spend with Wikipedia as I did) to give their input.