Where legacy airlines get into codeshare agreements and interlining deals to expand their networks, usually with alliance partners, low-cost carriers are going more down the route of digital hubs that allow travellers to book tickets on connecting flights across more than one airline.
In May Ryanair started to sell Air Europa long-haul flights through its website — meaning that passengers booking Ryanair flights to Madrid could also book onto a connecting Air Europa flight to 16 countries in North and South America. It was back in July 2015 that Michael O'Leary talked of basically becoming an OTA for flights, but a quick search does not bring up any results other than its own services.
And now easyJet has entered the game with its Worldwide by easyJet platform. The new system essentially acts as a search and book website for multiple destinations and uses Gatwick as a connecting airport hub. For example in this search, easyJet would operate the Munich-Gatwick leg and the traveller will connect to a Norwegian flight for Gatwick-New York (I chose those destinations at random, there might be ones with shorter waiting times).

Norwegian is one of the launch airline partners alongside WestJet, but talks are already in place with Middle Eastern and Far East carriers. Travellers can also book connecting easyJet flights and there are plans for more airport hubs at Milan Malpensa, Geneva, Amsterdam, Paris Charles De Gaulle and Barcelona. The site sells stand-alone tickets for airlines too, with Loganair the first to sign up.
Not only does this expand connections for Europeans but it grows the flight options for non-Europeans. A single transaction makes the booking process easier and offers a better airport experience for travellers.
However, this function is currently only available direct on the website. easyJet told me "Worldwide by easyJet cannot currently be booked by third parties or travel agents. It is available exclusively online and cannot be booked at the airport, through a travel agent or a partner airline's ticket office."
So there's a bit of time until we'll see this functionality in traditional corporate tools. But one thing's for sure - organisations from IATA to Skyscanner have foretold that airlines will become online retailers, especially as many reassess distribution strategies and how they will adopt NDC. Are websites like these the new OTAs?