The only thing we can do is loan him out and hope for the best. Or sell Bravo to City or something, which I doubt would happen.
As great as Bravo is, I'd still pick Stegen if I had to pick one to keep.
For any other goalkeeper in the world winning the CL and the Copa in your first season in a new country is earning your stripes and Mats is perfectly justified to be unhappy with the bench.
Or if a hard decision is made in regards to looking towards the future instead of the present. No matter how long you tell yourself Bravo can keep performing at the top level, you can add nine to that for Mats.
If he leaves this summer in search of playing time that'd hardly be a rushed decision. I'm sure you'd be tickled to have him as a bench option for the next seven years while Bravo continues starting most of our games but that's clearly not going to happen. I'd be shocked if we even manage to tie Mats down to a new contract at this rate.
A loan would probably be our best bet. Pull a Courtois on him. But even then, eventually the hard decision will have to be made eventually.
As great as Bravo is, I'd still pick Stegen if I had to pick one to keep.
Loan him to a La Liga team.
I keep on hearing about how good Ter Stegen would be "if". But from what I have seen in his play for Barca, he generally plays very well, except for some bad goals. Too many bad goals for a Barca #1 keeper. Whereas for Bravo, the performance is consistently excellent, with lots of hero stuff one-on-one, which is needed playing for this team.
Ter Stegen can go, Barca can bring in someone else to be #2.