Got a more verbose example of what could go wrong with an example so I can follow up with an example of how it could be done? Or more to the point. Got something I could look over on how you intended to model this in a way that wasn't based on syntax, so I can make a better example in context?
Template arguments, and especially the named variant, tend to have some loosely defined semantics apart from their syntactic identifier. These semantics are often explained in the template's documentation section, and can be mapped to a more formal ontology as in dbpedia [1].
A general schema for all templates could only reference things like the argument position or the argument name in an abstract way, but could not generally provide semantics for them. Note that this is different for things like images, where we can either define our own global schema, or directly reuse one from schema.org (e.g. http://schema.org/MediaObject), possibly with an additional extension for disambiguation purposes (/MW) as described in http://schema.org/docs/extension.html.
itemtype is really a meant for a real type, I'd really hate to see Microdata abused to the point where we abuse itemtype as a reference url and pretend that half of everything using itemtype comes from wiki syntax rather than a user entering microdata into WikiText.
Microdata items can be nested, so I don't see a problem with users or templates providing a mapping to more specific schemas like those of schema.org. Clashes of user-provided itemtypes with those used for editing purposes need to be prevented in the parser, but that is doable. Consumers are free to ignore itemtypes they don't know about, which is what Google etc are doing afaik- and what also motivated them to set up schema.org in the first place.
There might be ways to use schema information from the template documentation to add an additional, more general itemtype to the rendered template, but that is both still under development at WhatWG and not our highest priority right now.
Gabriel
[1]: http://mappings.dbpedia.org/index.php/How_to_edit_the_DBpedia_Ontology