Madrid would make easy sense just by having Aer Lingus operate the route rather than Iberia Express.
I also note that Düsseldorf got off to a relatively strong start, carrying more pax in May and June than the long-established Munich service (although I think the days in the month may have meant more services in June). It could be a possibility for growth.
You'd have to wonder though how much more demand there could be to the Barcelona region.