I've seen a few rumours of airlines like FR and EZY acting as feeders into DY, but I rather feel that these have been put out by DY to test the water rather than being active discussions. I can't imagine anything more nightmarish than FR and DY both strenuously denying that it wasn't their fault that your bags didn't make it to their destination.
If LX turned up, it would be a turn up. They're already at LHR and BHX, so adding BRS would make them quite concentrated in this corner of the UK. Maybe they'd get BM to wetlease, as SN does?
Would Jet2 want to operate out of BRS? Maybe in a W from one of their European destinations, but against EZY/FR, it feels like a long shot.
I look at my rough estimate of daily pax between the point-to-point airlines and the hub routes, and I think it's about 10:1, maybe a bit more. It feels like there's still opportunity in the European business/hub destinations.