This is not a request for a new field type but for a little text field enhancement, to make it act a bit similar to WordPress tags.
So the idea is to show (in a dropdown, select2 could be used) all the values of this field that exists in all the posts which this field is assigned to.
The implementation shouldn’t be very tricky. It shouldn’t have an influence on performance because it requires just a single meta query (if we include all the registered post types in the post_type WP_Query property).
You may ask what is a use case for such a feature.
So, sometimes we need a field that works like select, but we don’t know the values in advance. We want users to be able to define select options. So yes – for end-users it will work more like a select with custom values than a text field.