Custom Fields for the Link fieldtype

With Craft as dependent as it is on custom fields, it seems a bit incongruous to have the advanced fields of the Link fieldtype be a limited assortment of "hard-coded" options. I mean, what you do have absolutely does make sense, it just begs the question of why there isn't a "custom field" type of option.

IRL use cases for custom fields on links:

• Link style: Many times, a client will want to have the option to show a link in a button-style, rather than a "text link" style. It would simplify things if us developers could provide a dropdown field to select that on the link itself.
  
• Progressive enhancement: A link which goes to a "utility" page of some sort (login/register/search/subscribe/etc) by default, but if JS is enabled, the functionality of those pages gets loaded into a <dialog> instead.
• Marketing: Typically it's more user-friendly to allow content authors to add marketing URL params in a visual manner instead of a technical one. Having them type out "?ref=campaign_name" in the URL suffix field is a technical approach; allowing them to select (via a Relationship field) the associated Campaign entry is a visual approach.

Taken to the "extreme", perhaps links could benefit from being Entry Types themselves?

Originally posted by @Michael-Paragonn in https://github.com/craftcms/cms/issues/16268#issuecomment-2559004026

Start with the Link fieldtype entry point and review how its current advanced fields are defined. Compare the requested custom link fields and possible Entry Type approach, then define a focused scope and acceptance criteria for supporting author-defined link metadata.