smaller stations on either side of the Thameslink core, or e.g. somewhere in Cornwall or the Esk Valley to somewhere in Northumberland)
Thanks to
@splashoutradio's site, I've found that Camborne has passengers to Alnmouth, Berwick and Dunbar but not Morpeth (but there's no direct Camborne to Morpeth services as far as I can remember) and that Elstree & Borehamwood has passengers who've made journeys to every station on the Sutton loop, which I thought would have been some pairs with direct services and no passengers.
Newer stations – East Linton, Reston, Tottenham Court Road, Brent Cross West and Barking Riverside, at least – aren't showing up on the database for me.
Railalefan.co.uk's site doesnt show tied stations alphabetically, and Reston has a lot of stations tied on one journey to them, but Chester-le-Street, Northallerton, Dewsbury, Huddersfield, Stalybridge, Newton-le-Willows, Lea Green and Liverpool Lime Street aren't shown as having any passengers from Reston on the one direct train (19:03 from Edinburgh) whilst Hemel Hempstead and Daisy Hill have had at least one person travelling from Reston. My guess would be enthusiasts travelling back from Reston after the station opened.