Welcome to the forums.
As your post entails, traffic on Mapleton will become increasingly problematic as this retail area expands. This point was driven home to me just this last weekend. I was driving north on Mapleton and the traffic was lined up literally for two blocks on the inside lane as people were waiting to get into Cabela's.
What will happen if Costco moves to the Day & Ross site, or if IKEA locates in Mapleton Crossing? The situation will become increasingly dysfunctional. The city (and province) need to start planning for future traffic requirements now.
At the very least:
1- the Mapleton/TCH interchange will have to be upgraded in a manner similar to the recent upgrade of the Elmwood/TCH interchange,
2- Other traffic routing will have to be explored. As you have pointed out, Lady Ada has to be extended to LJR Blvd.
The below map depicts my understanding of what the city has in mind for new streets on the east side of Mapleton Road.
The dead end just behind the Mapleton Centre corresponds to where the new
la Promenade development is proposed. This new street will continue north in behind the Four Points Hotel, all the way to Day & Ross (where Costco is rumoured to want to go).