If I was on the Amex marketing team, I'd make three changes:
1) Change to a metal card. This isn't that expensive and it would create waves in the Canadian market where these cards are not common. This makes wayyyy more sense than huge signup bonuses.
2) Add in a restaurant bonus, similar to Cobalt.
3) Make the card FX free. This is something many people fixate on and I think people often overvalue.
I'd want people dropping metal platinum cards in front of their friends at restaurants. The no FX thing would give their friends an excuse to signup. This dynamic was going on with Chase Sapphire cards in the US and it helped Chase Sapphire become the standard in the premium card market - stealing Amex's market there. If I were Amex, I'd want to recreate the same atmosphere in Canada.
A 20% discount on the higher trims of Uber (Select, Black, etc) would also be much appreciated and I think it would fit nicely with the premium image Amex wants.
I have a platinum card I don't really use. But the reasons I don't use it are harder for Amex to fix:
1) Aeroplan miles aren't that useful (to me) due to a combination of poor availability and high fuel surcharges. This will likely get worse as AP and AC split. Perhaps add Westjet as a transfer partner?
2) Amex isn't accepted and/or tap doesn't work properly with Amex at half the places I use my card.
3) Canadian airports are generally okay, so I don't care that much about lounge access. If I lived in Philadelphia my view would be different.