Help needed for index field mapping after index creation

we have a scenario where we know set of fields that we need at the time of
index creation .This we know how do create field mapping at time of index
creation .

But ,
we have other scenario where users can create custom fields after index
creation and dont know what is the field properties of the field user
trying to create .Field properties like data type , IsGrouping and others

Not sure how to do it .? If you can help with us sample code , it will be
big help .

Also interested knowing thoughts on document schema this case

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I have the similar problem..