What will be a killer feature for Apple Pay and Apple Wallet?

Using a database, Apple Wallet will pick the card that gives you the best reward. For example, it will know that your Bank of America Visa will get you 2.625% on every purchase and 5.5% on food and travel. It will know that Discover for the 3rd quarter will get you 5% at gas stations up to $1000. It will know that your Citi Dividend will get you 5% at grocery stores in the 3rd quarter. (All these are just examples).

Whenever you are at a particular location (based on your location as tracked in iPhone etc) it will see that currently the best card to use is the Visa card and when yours Apple Pay it will pick that card.

It might even know that some other card could get you a better deal and give you the option to apply for it.

Then it will manage all the rewards, maximizing your returns on what you would spend anyway,