(Apologies for the long delay in actually posting this.) Here's the user interface breakthrough - a breakthrough at least until I try to implement and use it, at least - for node-centric community taxonomy management.
The actions we need to be able to propose are positioning terms in a hierarchy, merging terms (as synonym and as replacement name), adding synonyms, renaming the term, and endorsing any of these actions proposed by others.
The below would be needed for each term on a node. It's a lot. So the entire community managed taxonomy will be collapsed by default under each node, and each term within it would be collapsible.
rutabaga ˇ
miscellaneous > rutabaga [endorse term] (at this placement)
move under vegetables > cabbage [endorse]
merge into stuff > parsnips [endorse]
[_______________] [move under] [merge into] [merge onto]
Synonyms for rutabaga:
rutebega [endorse] [rename term as] [delete] [split into new term]
artichoke [endorse] [rename term as] [delete] [split into new term]
[_________] [add synonym] [rename term]
[edit term description]
[delete term]
(Proposed terms would follow the same format, in a lighter shade)
New term ˇ
[_______________] [Create term] (including terms in hierarchy as needed)
There is deliberately no option to [endorse term] without endorsing a particular placement (though in a no-hierarchy vocabulary [endorse current placement] would be replaced with [endorse term]). In multiple hierarchy vocabularies, there could be multiple variations of the line miscellaneous > rutabaga [endorse this placement]. Multiple hierarchies may require an [oppose placement] button, but I'm trying to get away without it.
Proposed synonyms, if any, would follow synonyms, in a lighter color (which would mean two scales of lightness for proposed synonyms on proposed terms), but the default will be for synonyms to be created immediately with just one vote.
The main action is in the "[____________] [move under] [merge into] [merge onto]" row, which will be a funky autocomplete field.
If you typed a path to a term into the textfield (autocompleting each step of the way, creating hierarchy by typing ">") for instance, "An > Existing > Term", the line "[____________] [move under] [merge into] [merge onto]" means, respectively:
Move rutabaga under An > Existing > Term
Merge rutabaga into An > Existing > Term (making rutabaga a synonym of An > Existing > Term)
Merge rutabaga onto An > Existing > Term (making Term a synonym of miscellaneous > rutabaga)
Note that "An > Existing > Term" could just as easily be "A > New > Term" in each of these cases (the hierarchy wouldn't be used in the case of making Term a synonym of miscellaneous > rutabaga)
Tool tips are supposed to work on form buttons. So the help text of [merge into] would be "Make suggestion at left a synonym of this term"; [merge into] would be "Make suggestion at left the new location and name of this term".