It would be great if Jack could stop God, and Sam and Dean wouldn’t have to become monsters and/or kill each other, and Cas won’t have to disappear.
If everything goes according to plan and Jack, strengthened by the bloody hearts he ate, is able to kill God, then maybe a happily-ever-after is in store.
The WInchesters did get their luck back after God removed his protection from them, so they have that going for them now that Jack is back with a mission.