When the template selector appears at the launch of Impress, being able to filter out 4:3 and wide screen templates would be handy. The filter drop down list would have entries for 'All', '4:3 Screen', and 'Wide Screen'. This drop down list can replace the disabled 'Filter by Application' drop down list.
Wouldn't add an explicit control for this property. The information is rather placed at the template's name.