In Europe there are about a dozen airports with good train stations and good connections. Since Skyscanner is about travel it would be good to include those in your search.
Could you build the option ‘direct + train’ as an extra checkbox on the search page?
Seen from outside of Europe you can puzzle a lot to reach your destination as close as possible with local connecting flights, but when train connections are included in the search travel can become way more convenient! And in case of business travel, more productive. Families travelling can strategically include the train to distract and entertain children on the last leg of the journey.
You can start with just one dozen airports with good train stations listed at https://www.raileurope.com/en/blog/best-airport-train-stations
This works best if the end destination is a city with a train station.
For example, when you search Groningen (GRQ) you also get Groningen (NS) with a train icon. The NS is the local train operator. Only train stations with a direct train connection to an airport with a good train station are suggested.
Currently, when you search ‘Delft’ you get “Amsterdam (AMS) 44kms from Delft” suggested. Make that into Delft (NS).
The feature requires you to add to the Skyscanner database all the cities with a direct connection to at least one of the dozen airports with good train stations.
While not only giving a good service to travellers, it will have an enormous impact on the environment, more than any individual or politician can achieve with such a relatively small effort.
Since the pandemic all railways sell digital train tickets based on a downloadable QR-code. How to get a small percentage on top of that is something to negotiate with the railways. Start with the Dutch railways I would suggest, they are easy to deal with. The margin for you might even be higher than on an airplane ticket.