Looking at what everyone else is doing / could do I think we should be going for 60,000.
There's too much worry over what the atmosphere would be like if we're averaging 53,000 say.
Barça "only" average 70k ish yet they're expanding to 100k capacity.
When Spurs is built, "if" L'pool expand and when Chelsea is redone that will mean all the other "big" clubs (plus West Ham
) will have 60k+ stadia.
It would be remiss to have a completely new build and position the capacity behind that level. We'd essentially be in the same position that we are now.
We're not a club that can charge huge ticket prices and we won't have 10-20k worth of corporate "customers" that would make up a capacity shortfall.
We need to have a capacity that will hold demand for big games. And this isn't our big games now - the stadium move is designed to have big games as late stages of the CL etc.