A bit of background: The Rebellions of 1837 were two armed uprisings that took place in the provinces of Lower Canada (Quebec today) and Upper Canada (Ontario today) which while fairly disjointed from one another were both motivated by the desire for political reform (which was to an extent fulfilled as a result by the granting of responsible government to the Canadas and eventually influencing the creation of Canada in 1867). The much larger of the two Rebellions was the one that occurred in Lower Canada with the number of Patriote armed rebels numbering in the low thousands with a good deal of public sympathy behind them eventually being defeated by larger forces of British Regulars and Canadian Militia. The Lower Canada Rebellion is fairly well known in Quebec today, in no small part due to it being put back in the public media by the Quebec Separatist movement in the latter part of the 20th century (The Spirit of '37). The Upper Canada Rebellion was much smaller and was defeated very quickly (although it subsequently led to the Patriot War) and is largely forgotten in Ontario today except for an old myth that when the rebels marched on British forces in Toronto and the first musket line dropped on its knees to fire, the rest of the rebels all thought they had been shot and fled, thus quickly ending the battle. All told the two rebellions were pretty small scale and only had a little over 100 combat deaths on both sides. Their was significant sympathy for the rebels in sections of the US public (the Hunter's Lodge was formed in Vermont and in 1838 launched a small invasion into Canada- which was defeated by British forces- with the intent of establishing a Republic of Canada). However the US did not directly intervene in the rebellions and ultimately arrested the defeated Lodge members when they retreated to New York (Out of curiosity does anyone actually know why the Us chose not to intervene in OTL?).
So the challenge is to turn this conflict, which is in OTL a historical footnote (even in Canada outside of Quebec it is pretty much forgotten) into a major international war. The most likely way to get this is by having another country intervene on the side of the rebels with the US being the best potential candidate (close proximity, public support for rebels, public resentment of the British) or possibly intervention by another European Power (were any of them in a position to overtly cross Britain in the 1830s?).
NOTE: The scenario does not necessarily have to end with Canadian independence (although it can); it can be a major war which the British are victorious (how would this change how British North America and the other Colonies would be governed in the aftermath?) or even where the Canadas are annexed by the US (how would this affect the development of the United States, especially through the Mexican-American War and the Free State vs Slave State tension that led to the civil war?).
Any ideas or insights would be appreciated.