They'd probably just build a smaller version that could be expanded quite easily like Brighton have done.
I'd be concerned about putting in a retractable roof tbh, dismantling and replacing it would be expensive making future expansion costly, unless cheaper versions and methods now exist. Too many clubs only look at the next 10 years, not the next 25. The Unigate site (one of the 3 apparently) is also quite small and boxed in by 2 rail lines east and west and the Westway to the north. Should go to the BBC site or Old Oak Common.