It would never get the green stamp of approval in this day and age. Plus the UK gov are far more interested in expanding Heathrow.Severnside airport does usually pop up now and then but it's a dead project that'll never happen but that doesn't stop it getting brought up usually in the argument to shut Cardiff down.
I always think even if money and the environment wasn’t an issue, the wind would be a massive problem on an island airport. The bridges are often closed to high sided vehicles in bad weather so I can’t imagine a runway open to the elements would fare much better.