It would be nice if the new MM has a suite of Legendary abilities that you add to monsters to give them some oomph for tactical encounters. Along with an extended section of advice for designing your own (No, not in the DMG - in the MM!). Offensive, Defensive AND Utility abilities so that you can make a regular creature into a "elite" version (like turning a regular crocodile in "Thousand Teeth", a regular goblin in "Captain Redhand" or a mundane vampire into "The Midnight Drinker" or such, or swap out abilities on an already legendary monster to give it an unexpected twist.
Some ideas:
OFFENSIVE
Backhand. A single melee attack against a PC that just attacked it, combined with a Shove.
Flatten (Costs two). A double-strength melee attack, combined with a Trip.
Overrun (Costs two). Move half movement rate, can pass through enemy space. Anyone passed through makes Dex save, on failure takes 1d8/size category (Medium is 1d8) and is knocked prone. Can't end in occupied space.
Smash. Target opponent at 1/2 hp or less, make melee attack that deals + Xd6 damage. Once per turn.
Sweep (Costs two). Dex save melee attack against all targets in Reach.
DEFENSIVE
Absorb (Costs 2). Reduce incoming damage type to 0, charge next attack with Xd6 of that energy type.
Counter (Costs 2). Opposed check, attack negated.
Deflect. Increase AC by +5 against attacks until next foe's turn.
Redirect. Attacker makes Int save. On failure, deal damage to target of monster's choice.
Resistance. Make save with +5/+10 modifier.
Remove Condition. Removes a condition - poisoned, stun, etc.
Rebuke (Costs 2). Reduce damage to 0, attacker makes save on failure takes Xd6 damage.
UTILITY
Evade. Move full movement rate.
Reposition. Move half movement rate, without provoking Opportunity attack.
Refocus (Costs 2). Heal Xd8 hit points.
Teleport (Uses two). teleport to unoccupied space within twice movement.
Superior Skill. Use skill (such as Perception)
Special. Activate special ability. May cost 2 actions.