Is it possible to have custom, default fields added to all new product types In Commerce 2?
Nothing that just works.

You could define them as a base field through code by defining them in hook_entity_base_field_info(), so for example how scheduler module does that. Then they will be non-configurable fields that are always there. If they're not multiple, they will even be added to the product base tables.

Those kind of fields are a bit tricky to uninstall.

The other option is implementing a hook_commerce_product_type_insert() and then copy the configurable fields you want from the default type and add them to that new bundle as well.