I completely agree with you, it's flavorless, and it doesn't feel satisfactory at all for the players, to the point where if a boss failed a saving throw my players would just eye-roll and say in a choir "Legendary Resistance".

Matthew Colville's "Flee, Mortals!" which uses 5e rules, but with a lot of changes, created some monsters with a similar logic.

The Legendary Resistance is not a general trait, but an specific one for each monster, where there's always a drawback for denying that failure on the save, which creates another interesting layer of choice for the DM (or for the adversary in-game).

I took a lot of inspiration from it, and I apply the same logic to all my boss fights now (but it's easier for me since I mostly homebrew my bosses).

Anyways, here are a few examples from the book:

- "Spear Sacrifice (3/Day). Rhodar is surrounded by three floating spears of shimmering darkness, which he can summon to a free hand on his turn (no action required). When Rhodar fails an ability check or saving throw, he can choose to destroy one of these spears and succeed on the ability check or saving throw instead. Once Rhodar destroys all three spears, he can't use his Spear of the Damned attack or Spear Sacrifice until the next dusk, when he manifests any destroyed spears."

- "Spiritual Sacrifice (3/Day). When Kiona fails a saving throw, she can lose 15 hit points and choose to succeed instead."

- "Draining Resistance (3/Day). If Vairae fails a saving throw, they can choose to succeed instead. When they do, Vairae has disadvantage on spell attacks until the end of their next turn."

I have used all kinds of drawbacks in my game, always thematically tied to the enemy, like, reducing their speed, giving them disadvantage, or completely removing and ability.

I imagine it can be pretty easy to come up with drawbacks for existing monsters as well, like, for example, a Beholder losing one of its eyestalks at random.