I wouldn't rule out EZY returning to SEN. Decisions made in the Summer of 2020 at a point in the pandemic when uncertainty was at its greatest may not look so good at the end of 2021 if signs of normality are on the horizon. For one thing RYR has left the scene and their arrival in Spring 2019 must have annoyed EZY greatly and was definitely a mistake on SEN's part with the benefit of hindsight. EZY could also drive an excellent bargain one would think and SEN might be a little more careful to avoid a repeat of the RYR situation.
EZY did invest in new aircraft at SEN and replaced the originally based A319s with A320s and based at least one A320neo there from Winter 2018 onwards and had four based aircraft for 2020 so I don't think their commitment up to then could be doubted.
I'm not saying I think this is a very likely scenario but it's certainly a possibility. I don't have great hopes of Wizz being a candidate as their LGW ambitions may annul any need for a SEN presence now.
I think Stobart would have been mistaken if they had built a terminal catering for anything other than a largely low cost customer base and compared to some European terminals built for their RYR traffic it is far from spartan. Shopping and catering tends to develop in line with passenger numbers and the 2 million passengers achieved in 2019 was insufficient to merit a greater selection and the quality of the catering was on a par with the offerings of STN and LTN in my experience.
There is at least one operator who I think may be interested in SEN's sun-hungry local catchment, which is not inconsiderable and is quite loyal to its airport, but it won't be helpful if I name names at this point.