That food looks good, imagine if they partnered with Uber or Grab so you can add to cart right below the picture.
Peer to Peer payments could have also been great, especially if you could check-out at a store by scanning a QR code to pay (think WeChat Pay, FairPrice in Singapore, or even Paypal's version of that).
Or even buying event tickets. They already have events on the platform, and they let you put targeted ads, but what about an integrated experience to purchase tickets right on the platform instead of there being an external link?
They could have done so much but the only major change/addition in recent years was Dating (a huge hit in countries that perceive Tinder as only for hookups) and those avatars that people use everywhere instead of text posts.
I agree that it would have been a good strategy, but that's (presumably) why they never did it.
Doubling down on becoming one of these "everything" apps could have been a good strategy. Become the app frontend for one of the less big food delivery companies in the FB app, tie in to payments. Perhaps even buy Square for Cash App and all the POS integrations to build a network of sellers, all tightly integrated from the consumer perspective into the Facebook app. I'd have hated it, but I suspect it could have worked.