WordPress multisite, allow non super admins to create sites

In multisite installation there already is a default admin and super-admin role, and if standard capabilities are different than what you want you can modify them:

A default set of capabilities is pre-assigned to each role, but other capabilites can be assigned or removed using the add_cap() and remove_cap() functions. New roles can be introduced or removed using the add_role() and remove_role() functions.

In this way you can customize user capabilities to your needs;

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As asked from toscho I’ll make an example:

you can specify new role and caps with add_role() (in ex. maior-admin)

add_action('after_setup_theme','maior_add_role_function');

function maior_add_role_function(){
$roles_set = get_option('my_roles_are_set');
if(!$roles_set){
    add_role(
'maior-admin',
'mj admin',
 array(
    'manage_sites' => true,
    'read'         => true,  
    'edit_posts'   => true,
    //set all capabilities needed - you could do this with get_role() but not so straightforward
)
);
    update_option('my_roles_are_set',true);
}
}