Having one game at Wembley a year has worked out quite well, but the novelty value will quickly wear off if one team has all their home games in London for a season. There is no way that the fans who turned up for the sole NFL game this season will put aside their respective allegiance and go to watch a London franchise team instead.
If there was a team from London, then could it not be possible to play a number of home games in a row, before embarking on a mini tour of the states? Would mean they wouldn't have to travel back and forth across the Atlantic every other week.
Also as has already been said, absolutely no chance that QPR will move to the Olympic stadium. More chance of them ground sharing with Chelsea.