It just won't happen

Vince Cable would have made a better chancellor of the exchequer than George Osborne in 2010. So would David Laws or Chris Huhne. But the Conservatives are the majority party in the Commons. There is no way that they would concede one of the great offices of state. 

They do things differently in Germany. The Free Democrats have provided both an economics minister (Count Lambsdorff) and a foreign minister (Guido Westerwelle) to coalition governments. 

With disquiet mounting on the "dry" side of the Conservative benches over what are seen to be too liberal policies and too many Liberal Democrats with government jobs blocking promotion opportunities for ambitious Tories, it is even less likely now that Cameron will improve the standing of LibDem ministers within the government. The best guess is that Hague (an ex-McKinsey man) will take over at the Treasury and that Osborne will be shuffled sideways rather than dropped.