Thinking about years they haven't covered, I think they've pretty much done all the 70s, although not sure if they've covered 1972.
Also not covered yet as far as I'm aware: 1988, 1990, 1992, 1993, 1997 to 1999, and anything after 2000 (but would understand if they might not find any mileage in the 20th century TOTPs).
1988 might be a decent-ish year to look at, although that was my prime TOTP time so might be a little biased, but I recall plenty of decent dancey stuff even before that year's summer - Coldcut, Bomb the Bass etc.