Easy, Mercy would still be vulnerable to CC. That means that even with 5 teammates down, she is still susceptible to getting shield bashed, slept, booped, etc. which would cancel her res before it completes, similar to how it can be cancelled now by the enemy team.
The Cancel Option would appear as it would, similarly to McCree. The difference there though, is that instead of it simply cancelling and expiring that ult, it would also boost Mercy in whatever direction she’s facing. 
Now that I’m thinking about it, to better control this and to add even more skill / complexity, a QOL change could be to allow her to be able to hold the boost in as long as the cancel button is pressed, which then launches her forward once it’s released, similar to Doomfist’s rocket punch. Maybe as a little perk, rez cancels that are held for each second, would get an additional 10 meter boost when released that is added to their initial 20 meters. So for an example:
Rez Cancel (pressed instead of being held) = Mercy is launched 20 meters
Rez Cancel (held for 1 second) = Mercy is launched 30 meters
Rez Cancel (held for 2 seconds) = Mercy is launched 40 meters
Rez Cancel (held for 3 seconds) = Mercy is launched 50 meters
I think this would be pretty fun as a compliment to Mercy’s bunny hop ability, allowing for her to capitalize on even greater movement opportunities, and rewarding better Mercy players for riskier rez cancels. 
~Sincerely Yours xoxo,
a Lover of True, Fair, and Fun Balance.
xavvypls 