The Ryanair operation at SEN was a success and according to Expressflight the 737-800 was only using 950m of runway to land. Taking this into consideration, SOU has huge potential for new Ryanair operations. This assumes the airport offers Ryanair the right deal.
This would be game changing for the airport if they actually get the runway extension and resurfacing done and stands that can handle a 737-800. I doubt it will be done by 2020, more like 2021.
One thing is for certain, I certainly wouldn't be trying to protect Flybe at this stage.