The main driver is likely to be easyJet, as has been the case for a number of years. They are part way through a five-year agreement with the airport which seems to include an additional based aircraft each year - summer 2017 will see number 14.
Whilst easyJet has been mainly responsible for BRS's phenomenal scheduled short haul network (for an airport of its size), and without which the airport would certainly not be looking at 10 mppa in the next couple of years or so, it almost certainly has restricted other scheduled airlines setting up, particularly locos.
Ryanair is the only other significant scheduled player with bmi regional a niche airline (with very expensive fares for the most part). KLM Cityhopper, Brussels Airlines (operated by bmi regional) and Aer Lingus Regional (Stobart) fly to hubs at AMS, BRU and DUB respectively (as does bmi regional to FRA and MUC). Wizz Air has four routes and WOW flies to Keflavik which is a hub for its North Amercan routes but beyond that (and it seems ever more likely that WOW will pull out at the end of the current winter) BRS's scheduled route network is meagre with airlines such as Aurigny and Blue Islands (now operated by Flybe).
Realistically then, any new scheduled airline would be competing for the most part on some routes already operated by easyJet or Ryanair (in some cases by both). Now whilst there still might be room for another loco, particularly on some of the sun routes, a potential entrant might decide that there would be a better return somewhere else where competition was less intense.
The airport CEO reckons that more than 8 million passengers will be handled in 2017. Looking at the programmes announced so far (and assuming that easyJet will announce that its existing schedule gaps in peak summer will be filled) it still looks as though another 200,000 passengers will have to be found from somewhere.
If it is accepted that neither easyJet nor Ryanair (nor Wizz for that matter) will announce anything significant for next summer beyond what is already in the public domain (and in easyJet's case including an assumption that those schedule gaps will be filled), then only a new scheduled airline for the airport, or a substantial rise in charter traffic, is likely to account for the additional passengers needed to meet the CEO's projection.