Oneworld member LAN's recently announced merger with Brazilian TAM (Star Alliance) is rumoured to take them out of oneworld leaving a huge gap in the network now that Mexicana has gone bust. Buying LAN could maybe keep oneworld's stronghold in South America.
As for buying Cathay, I am not sure it is for sale is it? The Chinese I doubt will give up their part and I doubt Swire will sell to BA. Walsh has already acknowledged that their wish list is just that; wishes. He knows that it will be impossible to get everything on that list.