A revised eticket email would be great.
Printed tickets or confirmations are useful to have when getting off the beaten track. I have been asked at immigration for onward tickets in a few countries - in the last couple of years Fiji, Palau, Papua New Guinea as well as at check in when using one way tickets when the country has an inward ticket requirement. CX at HKG were very hot on this. Sometimes showing them on your phone is fine, but paper always works better.