Crit range expands to 19, 20 (or 18, 19, 20 for tougher foe)
deals 5 or more ongoing damage of specific type with melee or ranged attack (bleed, fire, etc.)
on even roll target triggers a secondary special attack that deals typed damage (i.e. poison)
on either even rolls or 16+ target pops free and teleports or moves to somewhere nearby
On a miss target triggers a +2 to +5 defensive bonus against the missed target
Foe gets two or more of primary attack each strike (multi-limbed)
Foe's attack knocks you prone on 16+ or impairs you in similar way (nausea, weakness)
On 16+ strike foe is removed from play and reappears on its initiative action next round anywhere nearby (teleport or stealth effect) with stealth
Foe has a chain attack; each successful strike with a 16+ on the attack roll lets the foe strike another adjacent or engaged enemy
On 16+ the foe removes the target from play and places the target back on the battleground anywhere nearby or far away (shadow plane teleport your enemy to where you want them!)
Once per encounter (and probably after escalation 4+ this resets) deals double or triple base damage to foe as spite damage on even hit
Berserker rage triggers on 16+; monster's defenses drop by 4 but all melee damage is doubled and monster targets closest targets first, regardless of friend or foe possibly
target's melee or ranged attack pops a per-encounter secondary attack on a hit against MD that deals psychic damage and on a 16+ the target is dominated (difficult save to end) such that the controlling monster dictates either a move or action on dominated foe's round until the effect breaks.
Secondary attack triggered by melee attack is either paralysis or incorporeality (can't interact with objects or enemies physically, can still cast spells), with a variable save ends value.
Monster turns incorporeal on every odd escalation die value, gains immunity to physical attacks and damage but not spell damage
Swift Rush Attack...monster can move, attack, and on a 16+ may move and attack again (possibly up to X times if desired)
Pinning attack....on 16+ with melee or ranged attack target is pinned and immobile (save ends, or maybe STR check vs. appropriate DC ends)
Ganging up - monster works better in a group; gains an attack or damage bonus or effect bonus when two or more of its kind are adjacent to or engaged with the same target
Area attack - nearby foes, typically 1D3 or 1D4 targets, with a typed effect vs. PD or MD dealing damage and possibly ongoing damage
Dirt Bag Boss Monster Attack Special - Once per encounter, resetting on escalation die 4+ the boss monster can unleash one hellish attack, either area effect or melee/ranged which affects all targets nearby. Usually includes a save ending ongoing damage or impairing effect, and may include knock-down.