Most of what I'd love to see is undumped boards being found, ROM-dumped and preserved.
We've seen some pretty incredible things turn up over recent years like Hammer Away, Monkey Magic and Angler Dangler, I'm holding out hope for these someday:
Bouncer - the holiest of holy grails IMHO, we'll feel it on the Richter scale if that ever turns up. Lost vectors like Vertigo, Battle Star, Hovercraft, Scramble and Oops. Exidy protos like Snapper and Kreepy Krawlers Marble Man: Marble Madness II - heard a promising rumor. Any undumped DECO cassettes, especially Flashboy and The Tower. Vampire Gray Out Taito's Black Widow and Seafly Universal's Mrs. Dynamite and Limbo Eeek! Cutey Fatty Conquest / Journey Predators Zool Gremlins Surfer Fire & Forget II