There is actually plenty of spare capacity at SEN at present. Scheduled commercial movements at the 2019 summer peak are likely to be no more than 70-75 per day spread over the 17 hour scheduling period. They need to fill in some of the big activity gaps such as the 0800-1000 block. which probably.will have to be achieved with non-based aircraft. This could prove more difficult than attracting additional based units. The new Loganair routes are a step in the right direction though the volume may be small.
The airport already has planning permission for the terminal extension. As noted above surprise has been expressed that it was not completed in time for the arrival of Ryanair. There is space for six or so extra stands on the north apron. There always used to be stands on the north apron before the reconstruction work of two years ago so I would have thought that stands could be implemented as a permitted development, i.e. without the need for planning consent. Or they could "borrow" a couple of stands from the Stobart Jet Centre. They would need to accept a coaching operation but having done that there is scope for expanding the based fleet without undue difficulty. Provided of course they can manage the terminal issues with the early morning wave of departures rather better than they achieved on Good Friday.
In short the building blocks are already in place for a potentially significant increase in passenger numbers without the need to run the gauntlet of the planning system