Few days ago I made a PR #4316 for unifying the way joomla responds for duplicate alias in com_content. Although this unifies, at some extend, the user experience in articles creation the rest of a joomla installation might still behave differently when you input (or leave it blank) an alias that might already exists.
One posible site wide solution will be the introduction of a new field type, lets call it Alias, that will take care of the uniqueness of the alias.
What I am thinking is four changes:
1. Create a new field alias.php at legacy/form/field
2. In xxxx.xml of every component that incorporates an alias field so it uses the new one
3. In legacy/controller/form.php to introduce some ajax check (e.g.: on save) for the given or blank alias
4. In legacy/model/form.php the actual code that the ajax calls

This is what I think that the alias field should consists of (this is for articles):

<field name="alias" type="text" label="JFIELD_ALIAS_LABEL"
        itemid="id"
        itemalias="alias"
        itemtable="content"
        categoryid="id"
        categoryalias="alias"
        categorytable="categories"
        description="JFIELD_ALIAS_DESC"
        hint="JFIELD_ALIAS_PLACEHOLDER"
        size="40" />

Do you think is a good idea?
Will it break the universe harmony?
Is it useless?
Some input before I spent some time coding this will be highly appreciated.