You can use a schemaless changeset

{%MyStruct{}, %{custom_field: :any}}
|> Ecto.Changeset.cast(%{"custom_field" => "1"}, [:custom_field])
|> Ecto.Changeset.apply_changes()

Ecto can work for some inputs I found I quickly outgrew it and wrote GitHub - Adzz/data_schema: Declarative schemas for data transformations. too. If you need to map data that isn’t essentially a map or a list then ecto schemas aren’t the solution imo.

I’m assuming this is for similar reasons as to why you can’t use nil for embeds_many—which came up again recently—that embeds are meant to be treated the same as regular relations. I think your only option if you want to keep using Ecto is to create your own type. Otherwise you could check out Drops which is a more general schema library.

But now actually answering that part, and expanding on what @sodapopcan . Understand that althought schemas and changesets are just mapping abstractions. both schema and embedded schemas are expected to be at some point dumped to a database.
Since that is not your intention, i’d recommend to use schemaless changeset(you can use that with structs or plain simple maps).
in case you need features like nesting, relations and some more complex stuff that doesn’t work with schemaless changeset i’d do something like:

@default [virtual: :true]
schema "" do
  field :custom_field1, :any, @default
  field :custom_field2, :any, @default
  ...
end

Thank you all for answering here!!
@cevado yes… Actually this variable will be inside an assoc of the main struct
I will try then to find a better way to check these types
Maybe Drop is a nice candidate! Thx @sodapopcan !!