The ideal scenario sees Old Trafford rebuilt.
Ideally in that time we would go wherever Manchester United go - be that remaining at OT during redevelopment or the Etihad. I wouldn't be averse to a Cardiff or London final though that said. The loyalty to Old Trafford has been worth it, its arguably one of the sports few long term partnerships.
This really hinges on Sir Jim Ratcliffe's ownership pushing the football club in that direction of redevelopment. If he can't get it going then there is going to come a point where it isn't good enough. I've sat in various parts of old trafford and whilst the bowl itself is very good, potentially the best in the country tied with Cardiff, the rest of the ground is really dire. Its very tired and the video on page 1 epitomises the state of things.