"Shameless The opera" on opening night perhaps?
I'm afraid I'm afraid I can't take this proposal seriously.
A state of the art performance area would be an asset but at significant public expense. For this to work it would need to be capable of housing not just opera companies but large scale performances of other works and that is where the idea begins to unravel as Manchester and Salford possess these.
As a tourist draw it is barely on the scale, so little benefit for local employment or residents. It is hardly likely to attract West end Theatre productions which would bring in tourism, Cameron Mc and Lloyd-Webber are hardly likely to move their productions to Manchester, they have some significant financial challenges in London based competiton for audiences.
To my mind this is far too dependent on public subsidy to make it from idea to reality.
I can see where this comes from, the same team with the London 2012 hole in the ground and a budget that omitted VAT

which still needs be funded somehow, if anyone can arrive at anything approaching realistic final cost.
Opera is hugely expensive to stage and appeals to a small audience; show me an Opera house anywhere in the world that doesn't require substantial subsidy. But that inconvenient truth will not prevent the quick fix short term politicians from doing it!
In the real world, the casino was a fiasco and no political party has an alternative. It doesn't appear that anyone can think of privately funded replacement for the land which would be sufficient attraction to bring in large numbers regularly to fill hotels etc. Why attempt to pour even more public money away.The local area has lost it's labour intensive industry and with it much more, as Phil has pointed out"boom" in one sector doesn't produce local benefit. The idea of an opera complex producing any "boom" is rather stretching credibility!