We don't know if and when we'll revisit the idea, but we've previously explored the possibility of exposing some of the advanced schema constraint options (XSD schema facets) in the User Interface.
For min/max constraints on a multivalue select, it might look something like this.
Aside from multi-value, there's a relationship between "Mandatory" and "Minimum" that we considered in the design. Technically the Mandatory boolean option could be removed as it would be represented in a minimum of 1 (or more). However, this seemed like a common expectation among existing Tridion Schema designers.
Would visualizing the minimum and maximum (min and max) constraints in the UI better represent this idea? Or is it about having the Allowed Multiple Items checkbox to better accurately reflect the Schema values?
Regardless of the visible setting for Multiple Values, does the Schema preserve its min and max settings for that field when changed?