As has been voiced in several other threads, the selection of tracks (and cars) in GT5 is very disapointing, so there are lots of track that could be included as DLC. But if I had to pick 5 Real World (RW) circuits they would be:
5 points: Autodromo do Algarve, Portugal
4 points: Circuito Ascari (Ascari Race Resort), Spain
3 points: Lago Potrero de los Funes, Argentina
2 points: Circuit de Spa-Francorchamps (with original bus stop chicane from 2002), Belgium
1 point: Silverstone (Bridge layout), UK
If PD do decide to include real world circuits like Spa, Silverstone and Hockenheim it would be great if they could include the older versions (Spa - Bus Stop chicane, Silverstone - Bridge corner, Hockenheim - long confiuration) as alternative versions of the circuit as they have done with Le Mans by including versions of the track with and without the chicanes on the mulsane straight. Fingers crossed this makes it as DLC and they don't over price it.
James