You could use a propertylist with the url and the option to set the rel attribute

For a single "EPiServer.Url" property?

[ContentType(GUID = "...")]
public class SomeRandomPage : PageData {
    [Display(Order = 100)]
    public virtual Url PageLink { get; set; }
}

hmmm, that would be overkill I guess. You could limit the amount that could be added to one though ;) But I think might be the easiest way

Cannot accept this solution sadly. The customer has already 5 sites in production that needs this (single solution). And I don't want to force the customer to change everywhere.

Can't you use 

https://world.episerver.com/documentation/developer-guides/CMS/Content/Properties/Custom-properties/

And inherit from Url? But I don't know if it's a propertystring or something else as a base property.

A custom property, even if you would inherit from Url would kinda have the same difficulties, as it would still be a different property? In your situation, with sites already in production, you could add a seperate property in which to select the rel and render them together in your view. It is/feels a bit hacky though

Yes. But in my maybe faulty logic, since it still inherits from Url it would easyly be converted to the new type?

I that seems to be the only simple solution sadly. Thanks!

For future refrence. How do I extend Url class?

How do I extend Url class? Anyone?

I want a dropdown or textbox to set the rel attribute.

Can you somehow do a editor descriptor and add the new field?

[EditorDescriptorRegistration(TargetType = typeof (Url))]
    public class UrlEditorDescriptor : EditorDescriptor
    {
        public override void ModifyMetadata(ExtendedMetadata metadata, IEnumerable<Attribute> attributes)
        {
            base.ModifyMetadata(metadata, attributes);

            
        }
    }

There are some older posts that will give you some tips on how you could do it, but dojo will be involved for sure.... So personally I would pick an easier way.  An old thread, an old post from Anders