Considering that BMI have stuck it out on the SOU to Munich route for nearly two years now and with consistent loads the airline must be happy with the economics of the route to have sustained it to this point.
With the apparent success of SOU to Munich I am rather surprised that BMI haven't announced any other route or routes even from Southampton, the airline could possibly fly to business destinations currently unserved from the airport and BMI could have a similar but smaller network to their Bristol operation.
Possible destinations from BMI could be Brussels, Copenhagen, Frankfurt, Milan, Zurich all feeding into Star Alliance hubs except for Milan.
BMI could set up quite a niche network from Southampton without having to operate any routes that would overlap or be in direct competition with Flybe.