My definition of gimmick in this case is an innovative feature and rarely seen feature which does not demand to become standard.
In this case the word "gimmick" does not carry a negative connotation.
Banzai Run's vertical playfield
Pinball 2000-type holograms (Revenge from Mars and Star Wars Episode 1)
A projector in the apron as in Stranger Things Premium and LE
Triggers (Demolition Man)
Another mini-playfield like, but not necessarily identical to, The Power from Twilight Zone
A ceramic ball like the one in Twilight Zone and Avatar LE
A video mode with a level editor like Bride of Pinbot 2.0
Magna-slings in Ghostbusters Premium and LE
Rising and Lowering pop bumper from Cirqus Voltaire and others
Another mini-playfield like, but not necessarily identical to, The Sanctum from The Shadow
Another marionette head like Funhouse or Road Show
Under-the-playfield holograms like in Creature from the Black Lagoon
Mist Multiball like in Bram Stoker's Dracula
An industrial strength fan like Whirlwind or Twister
A ramp like Iceman's ramp from X-men Premium and LE
A ball saver like the one in Goldeneye or Alice Coopers Nightmare Castle
A central magna-save like the one in World Cup Soccer
A mini-pinball like in Family Guy or Shrek
A pinball machine with a board game woven into the rules like Safe Cracker
A pinball machine that dispenses something (a token, say) like Safe Cracker
Flipper blinders, like The Who's Tommy Pinball Wizard
NBA Fastbreak's unusually low scoring system
A mini-game in the backbox like NBA Fastbreak or Terminator 3
Upside-down flippers like the ones on Black Belt
Scissor flippers
Cocktail-sized pinball machine
A two-headed pinball machine like Joust
Something that can knock the ball off a habitrail like WWE Wrestlemania LE
A combination pinball-video game like Baby Pac-Man
A 3D spinner like Mini-me in Austin Powers
Stuff happening between the flippers and the apron like Blackwater 100
A molded 3D playfield like Orbitor 1
Bells and Chimes like Time Machine
Glo-Balls like in Viper Night Drivin'
The star field from JJP Pirates of the Caribbean
Varkon-style pinball machine (vertical with mirror)
Big Flippers from Big Flipper
A pop bumper between the flippers
A pop bumper instead of a slingshot, like on Rick and Morty
Player-controlled slingshots like on WWE Wrestlemania
Banana Flippers
Edit: This isn't a list of gimmicks I'd like to see return, these are just all the gimmicks I can think of.
Edit 2:
Modes that you navigate like a map as in Stern Jurassic Park
Plundering as in Jersey Jack Pirates of the Caribbean
A multiball you qualify with switch hits as in Elvira's House of Horrors
Stern's Meteor has a thing where you're incentivized to get all but one of the drop targets down, or as close to that as you can get.
Napa, CA
Mississauga, ON
Unknown
. I haven't played that, so maybe I have to retract that last statement someday.
