Napoleon dies in Russia, ending the Napoleonic Wars several years early, leaving Britain free to turn significant strength toward the war with America.
Basically, just have how Decades of Darkness play out. Jefferson dies, which leaves Madison in the presidency. Then, have everything go wrong for the United States, and have the Northeast secede.
These are both good points in how the war could alternatively play out with a set of good PODs.
And have everything go right for the British. For example, have them sink a majority of America's fleet out in the Atlantic, and have them pull a Washington on the Southern coastal cities.
The end result is a US without its financial centers, and one that has slaveholders as a majority.
Well look at OTL, everything that really could go wrong almost did. The American performance can be chalked up to gross incompetence and absolutely abysmal planning by the War Department. On the seas the US Fleet was basically a non-entity with the Royal Navy able to come and go along the US coast as it pleased.
It was on the Great Lakes where the American Navy actually made a difference, especially Lake Erie. The best POD's early on would revolve around British victories on those lakes. On Lake Erie preventing Barclay from being wounded in OTL's battle and you most likely pull off a win (especially if
Charlotte and
Detroit don't collide). The number of times Yeo and Chauncey failed to come to blows on Lake Ontario (with a list of absurd excuses as long as you can imagine) borders on ridiculous. However, the relative strengths and weaknesses of each squadron are somewhat known in 1813 and if the British are being more aggressive you just might get Yeo pressured into actually seeking battle.
However, let's just say that the Americans under Dearborn decide to attack Kingston instead of York. This would force some sort of decisive battle in late May on Lake Ontario. The British have over 2,000 men at Kingston OTL (maybe more with an 1812 POD and direct threat to Kingston) and the Americans hoped to have 7,000, but in reality due to mismanagement and disease OTL had less than 6,000. Let's say they arrive at an even 6,000 number here.
Now here Kingston will still be defended by batteries, forts, and blockhouses, but Yeo's squadron is also in port with the newly constructed
Wolfe but maybe not the
Isaac Brock (no attack on York means she isn't burned like OTL).
Chauncey has more ships, but the weight of armament favors the British 127 (or 106 subtract
Isaac Brock and assuming Brock has the same armament as
Wolfe) for the British (especially if some schooners are detached from the American squadron to protect the transports) and 95 for the Americans. Now both Chauncey and Yeo would be forced to seek battle here, Chauncey to protect the landing forces and Yeo to try and destroy them.
Personally I would give the advantage to Yeo, but even if he is bested he can fall back under the guns of Kingston while having more than likely inflicted considerable damage on Chauncey's fleet. On land the Americans are under the direction of Dearborn, and depending on the events on the Lake are either compelled to assault the works, or settle in for a siege. However, with a desperate fight on the lake Dearborn will probably be compelled to assault the works at Kingston. However, without the support of Chauncey's squadron it will be dicey and since he must divide his force to assault Kingston from both ends, will be in a difficult position.
Let us say then that the battle on the Lake is a defeat for the Americans due to the superior British weight in guns with Yeo choosing to engage on the lakes but feigning retreat leading Chauncey into the guns on Mississauga Point. On land perhaps the Americans succeed in driving the defenders from Point Henry while they are stymied around the town. The defeat of Chauncey's ships would compel an America evacuation but would likely result in the capture of at least one of the forces on land (we'll say the men at Point Henry are successfully evacuated while those assaulting Kingston proper are compelled to surrender thanks to the presence of Yeo's ships).
Chauncey suffers the loss of his flag, as well as
Sylph and
Scourge being captured. The smaller schooners
Julia,
Growler, and
Ontario being captured or sunk. The British however lose
Royal George as it strikes the shoals in Cataraqui Bay while the
Wolfe is seriously damaged in the fighting with
Madison and the schooner
Sir Sydney Smith is lost as well.
This leaves the British with a temporary disadvantage in the immediate aftermath in terms of ships (but assuming they are reinforced by
Isaac Brock then they have the advantage again) but with a crushing victory on land with perhaps over some 2,000 Americans killed, wounded or captured. This throws the Americans in the region firmly on the defensive and with reinforcement to Prevost's forces would leave Sackett's Harbor open to a like assault.
However the British may next devote their energies to assaults on Lake Champlain which may be better from the strategic POV.
For the aftermath of this 1813 battle let us assume then that Napoleon is indeed dead in Russia, freeing up British troops earlier. That would leave more time to construct a fleet on Lake Champlain which makes an alt-Plattsburgh a much less close affair. Perhaps Sackett's Harbor falls to a British assault as well (with the construction of larger vessels at Kingston they outnumber and outgun the Americans).
More speculation is necessary yes but after some disaster at Kingston the British options on Lake Ontario are pretty good, enough to really turn the strategic situation around.