Alright, I've been thinking about the kind of DLC that I've want to see included, and I've come up with a list. Some of these might not be obvious choices, but I will explain why later.
Circuits
Österreichring - from the halycon days of the turbo era.
Kyalami - the 1961-1988 version, of course.
Long Beach - it was always one of the better street circuits.
Montjuïc Park - one of the most awesome circuits ever built.
Oscar Alfredo Gálvez - the 1974-1981 version, not the glorified go-kart circuit used from 1995 to 1998.
Imola - using the pre-1994 layout.
Cars
The Gentlemen Racers
Alfa Romeo 158/159 Alfetta - the car Juan Manuel Fangio won his first World Championship in.
Ferrari 246 F1 - Mike Hawthorn's championship-winning car.
Lotus 49 - one of the most important racing cars ever built.
McLaren M7A - the car that scored McLaren's first win.
Matra MS80 - Jackie Stewart's car from 1969.
The Decade of Downforce
Lotus 72 - won three World Championship (including, sadly, Jochen Rindt's).
Tyrrell 003 - the first Tyrrell to win a championship.
Tyrrell P34 - because it has six wheels.
McLaren M27 - the car James Hunt won with.
Lotus 78 the first ground-effect car.
The Age of Turbos
Brabham BT52 - one of the most unusual-looking (and best) cars ever raced.
Williams FW07 - won Williams' first championships.
Toleman TG184 - the car Ayrton Senna took to second place in Monaco.
Renault RE30 - the car Alain Prost used in his first victory.
Williams FW11 - complete with the Red 5.
The 1990s
Leyton House GC91 - one of the first cars Adrian Newey designed.
Benetton B194 - from Schumacher's first World Championship.
McLaren MP/4-13 - the car Mika Hakkinen used to beat Schumacher.
Jordan 199 - Eddie Jordan proved that privateers could still be competitive.
Stewart SF3 - one of the last big surprises of the 1990s.
The Nearest Past
Honda RA107 - one of the worst cars in recent memory ...
Brawn BGP-001 - ... juxtaposed against one of the best.
Renault R25 - Fernando Alonso's first title-winner.
Ferrari F2002 - the car Schumacher used to win (almost) everything.
Williams FW26 - won Williams' last race before 2012.