Depends if you mean just the 1998 amalgamation or also the 1953 formation of Metropolitan Toronto. I think 1953 rather than 1998 was Toronto's "1898 moment" (formation of Greater NYC).
So, to get a scenario with very different paths between Toronto's old city and the boroughs, the point of divergence would essentially have to start in the 1950s then?
So, some wildly speculative "alternate history (a genre of fiction I used to like to read)" scenarios here for Toronto's suburbs:
After Toronto stops annexing suburbs in the 20th century, with no Metro Toronto formed in the 1950s (or perhaps delayed so that the boroughs have times to develop a few generation(s) of identity and self-determination), the towns and townships surrounding the old city now are the captains of their own fates.
North York -- Mel Lastman's plan for the city with heart can go as planned, with a thought that it could rival downtown. If his plan succeeds, could we even have North York center's relationship to downtown itself what La Défense is to Paris. In this hypothetical, North York contains more of the shiny, tall skyscrapers while the Old city and downtown retains much more old, historical architecture, which then is also better preserved.
Scarborough-- The town hitches its wagon to heavy industry and manufacturing, as demonstrated by the Golden Mile of Industry. But deindustrialization hits hard and it turns Rust Belt. There isn't much growth, and Scarborough in addition to being the leafiest, greeniest part of the city then later becomes also the most sparsely populated. Perhaps the post 1970s wave of immigration bypasses it, and rather than being one of the more diverse, it's one of the least instead. Alternatively, it does receive lots of settlement and immigration while the going is still good in the 60s and 70s but then flips to a negative growth rate. It also still continues to get more ignored by the city and province (a more Staten-Island-like Scarborough in some ways). Could a similar thing happen with Etobicoke?
Also, if a Rust-belt Scarborough or Etobicoke borders a wealthier North York and Yonge Corridor all the way to Downtown, it would not be good for social and economic inequality. Perhaps in a more divided Toronto, there is higher crime?
Whither Greenbelt? Another scenario would be if Toronto and its suburbs had less coordination, we might just have southern Ontario's answer to California's Inland empire. The way those counties making up the Inland Empire contains LA's suburbs but also spreads into agricultural country and ultimately to wilderness and desert, likewise Toronto's suburbs reach into agricultural country and ultimately to wilderness and boreal forest, bordering central Ontario in a similar way.