I am trying to create subtypes with default values for ArcGIS Online. So far, I've not gotten success when published to ArcGIS Online. Is it true to assume that ArcGIS Online doesn't support subtypes and default values?
To get dropdowns and default values for new features, try creating a feature template for each subtype.
In ArcMap, right click the current template in the Create Features window, and select copy. Then adjust the values accordingly.
Or in ArcGIS Online, after opening the add new features panel, hit Manage at the bottom right of the panel, then hit Add new Type of Feature.
This support article may also be helpful: http://support.esri.com/technical-article/000012361