The best way to get an idea added is to write a good complete patch.  2nd best is to provide constructive input on ideas that are already here.  3rd is to illustrate how you would hope to use such a feature.  Include ideas on how the usage/help/error display would work.

At the risk of repeating myself, I'm still not convinced that being a 'native type' makes any difference.  Argparse does not support any native type, at least not directly.