Don't think it really needs anything more than you said earlier:
amenity=fountain + fountain=decorative / utility / drinking
should cover it?
No, this is not enough to cover the features that are currently tagged,
thus this would be a regression.
Currently you can tag nasone, toret, roman_wolf and wallace which are
specific models/styles of drinking fountains. For example, nasone is a
particular type of fountain present in Rome; all fountains of this type
look the same. Deleting information about them being a nasone and simply
tagging them as fountain=drinking would mean losing information; this
means that if you're in Rome and you check for the presence of drinking
fountains you cannot discern which ones are a nasone and which ones are
not. Having lived in Rome I can tell you that this is important
information and that people actually do search for this kind of things
(as long as it's easy enough).
These should be, in my opinion, fountain=drinking,
new_key_describing_fountain_style=nasone.
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