One answer might be "infrastructure". Infrastructure in space will become big if space becomes big (beyond launching telecom, observation and science - which are small for now, compared to their launch potential.) SpaceX will need infrastructure itself - like refueling, landing stuff in exotic places, cargo delivery to other spacecraft, de-orbiting stuff, spacecraft to spacecraft networking, bringing cargo back to earth, carrying / delivering crew around, spacecraft repair, emergency access, etc.
A bit like Amazon then: if SpaceX needs some capability in their grand plan to Mars or whatever, they could try and package that as a commercial service after using the bare bones version for themselves.
A bit like Amazon then: if SpaceX needs some capability in their grand plan to Mars or whatever, they could try and package that as a commercial service after using the bare bones version for themselves.