Two things I would love to see in ESO are a playable Maormer race and a Mariner class with skill lines themed around Ships, Water magic, and semiaquatic Reef summons. We may never get those things. There are some sensible arguments against adding playable Maormer (like in-game dialogue), and demand for Mariner was much lower than other classes (Mechanist, Bard, Monk) in my poll on the topic.

But what if those two ideas have a shiny new baby idea? A Maormer Mariner, implemented as a Companion? That would be a perfect addition to the upcoming High Seas event. Companion skill lines are far simpler than full classes, and plenty of suitable Water Magic abilities already exist in-game. A playable class could always be fleshed out later when resources permit.

And why stop there? The same idea can be applied to other common new class requests. If dev resources are too slim for a full class, or if there are other reasons a certain class is being avoided, it takes a lot less resources and/or might solve the problem to build a Companion of that class instead. A Companion Monk (who always remains unarmed) would be easier to design than doing both the Monk class and Unarmed weapon line that are mutually difficult to implement satisfyingly without the other. The musical complexity & noise potential of a Bard class could be streamlined as a Companion. A Mechanist companion with automaton summons could answer a popular request, at least in a subset of content, which may be the best place for it (if, for example, you wanted to keep Dwemer automatons out of Cyrodiil as a design preference or whatever).

TL;DR: Companions could be designed with new classes (or races) that are not playable (yet) and that would be cool. This may even offer a superior implementation for some popular ideas.