In addition to EBD (which goes without saying) I'd definitely buy a Harlem Globetrotters playfield and plastics sets if they ever get the licensing ironed out and can produce them.
I would also buy another Bally Star Trek but it would have to be revised with red inserts for the specials instead of the white inserts that were used last time.
If they made Fathoms and Playboys again I'd probably have to get at least one more of them too.
Two prime Bally SS playfields, plastics and backglasses that would sell like hotcakes but involve complicated licensing constraints would be Kiss and Rolling Stones. I'd definitely buy a few of each (especially Rolling Stones) if they ever became available.