We have BeXmlSerializable, BeDataContractSerializable and BeBinarySerializable, and since 8.8 the first two accept EquivalencyOptions. There is no equivalent for System.Text.Json, the serializer most codebases use today.

Proposed API:

subject.Should().BeJsonSerializable<MyDto>();
subject.Should().BeJsonSerializable<MyDto>(options => options.Excluding(d => d.Etag));

Implementation: serialize with System.Text.Json, deserialize, compare via BeEquivalentTo. An overload accepting JsonSerializerOptions would cover custom converters.