Now most interesting for me to define optional (nullable fields), so, instead of just:

"type": "string",

Allow to set something like:

"type" : [ "null", "string" ],
"default": null

But there are also more interesting cases when e.g. instead of just int I would like to use logicalType date.

Please allow providing some functional interface to override fixed mechanism by providing an array of mappers, or just callable.