It is theoretically possible, but I’d not recommend it.
Extracting references from rich text is a PITA.
References are great for adhoc relations where you don’t need to leverage them elsewhere.
As soon as you want to build upon them (formulas, automations, lookups) they become less useful.
Anything you built might get superseded by genuine polymorphism at some point so your work would be wasted.
For the time being, if you must have links to many dbs, it’s probably better to just use multiple relations and optimise the UX/UI with thoughtful relation views and field hiding.
If you can afford to wait a while, that’s probably going to yield the best result
I, too, wonder if the concept of ‘tags’ wouldn’t accomplish a lot here. Creating a new entity type and then associating it with lots of other entity types (if I understand that’s what’s being suggested) seems a little like overkill for many use cases, if a person could just create some tags that can be applied all over the place in Fibery.