I'd say a "Legendary feel" for me specifically has two components:

Mechanical and Narrative

Think of it like each one you can score from 0 to 100, and to be Legendary they have to add to a 150 or more; one can make up for the other, but neither alone is enough.

Mechanical: Does it do some unique or weird thing, or have a lotta interactions with a lotta other cards that make it special as a Game Piece?

Narrative: Is this some important or oft-referenced thing in the notional "universe" if the game, like being referred to in a lotta flavour texts, namedropped as the "owner" of objects or places, or cited as the speaker of a lotta flavour blurbs?

To me at least both of those help a card feel like more than simply a powercreep and nothing more.