With POD no later than 1700, make the entire North America English speakers as non-rhotic instead of rhotic in OTL. What would be the consequences in American and Canadian culture if their English dialects are mainly non-rhotic?
It would require a new wave of British immigrants numerically overwhelming the existing population, whose ancestors came in before r-dropping was common. Trouble is, in the early 18c, when such immigration would still have been plausible, only the prestige accents in Britain were non-rhotic.
There are also some American accents that are non-rhotic: https://en.wikipedia.org/wiki/Rhoticity_in_English#United_States
Strengthen the influence of these dialects at an early point of time, and you might get there.