Those are some good ideas Linde, but they really don't give the landed regent any reason not to exploit, a single prosperity hit is easily overcome.
I do, however like the contest to affect source levels, that's an idea worth perusing. However, making it happen every turn is putting an awful burden on the wizards, especially as there isn't any real detrimental effect on anybody else.
One idea I was toying with was that when the number of extra GB equaled the Maximum Source potential, then the max potential would be permanently reduced. Perhaps, as a way of making it a little less onerous on the source holder, we could use that sort of measure, so that the free contest happens whenever the extra GB equals a multiple of the MSP. If the amounts of cash are too low, then the free contest could be triggered at 1/4, 1/2, 3/4 of the MSP and so on.