IOMX
If you want a regular scheduled service to a wide variety of destinations then frankly you need a nationalised and subsidised local carrier. And you probably need to pull easyJet's route authority, because even though your local carrier may not make money (it will lose quite a lot), to minimise losses and keep frequency, you'd need to capture a % of those who would fly easyJet, as you will need volume. As they won't choose an ATR over an A319 if they're semi-relaxed about their schedule and reasonably price sensitive, then they'll fly elsewhere, and probably on easyJet!
Hence for what you want, you actually need to get rid of easyJet as it's going to destroy any profits your airline could hope for in good times on higher volume routes, and it needs those to cross subsidise the lower volume ones. The days of G-OJET or G-MIMA popping over to LHR 3 x daily and charging and arm and a leg for the privilege aren't something I think we should aspire to. There's no simple win. I mean look at Aurigny, they only still exist because A319s can't get into GCI with a commercial load.
The use of "filler" is a misnomer, every off peak slot is a filler by that definition and if they don't bring the bacon home, they go elsewhere.