Avengers Theory: How Kate Bishop Appears In Endgame
With Clint having supposedly retired as Hawkeye to take care of his family - and with there being a tragic chance they all died in the events of Infinity War - the time for Barton to feature in the MCU as Ronin instead of Hawkeye has never been so well primed. After all, Clint promised to retire the Hawkeye identity, but he said nothing about retiring entirely. With Barton being characterised so strongly as wanting to protect the innocent, it's heavily likely that his reaction to the events of Infinity War is enough to totally break his resolve to stay away from super-heroics for good.
In this vein, it's also worth considering that the Young Avengers - or at least Bishop - would serve as an effective way to patch over the tensions still existing from Civil War, through the combined desire of all factions of superheroes to protect the 'next generation' of hero as best they can. Given the aftermath of Civil War is still very clearly affecting the lives of the Avengers, finding something that brings them together - aside from grief - seems like an effective way to reassemble a heavily divided team.
All rumours and speculation aside, with the limited cast Avengers: Endgame is working with, the likelihood of them introducing new faces to the MCU has skyrocketed, something that fans have to be at least a little hype about.