Not exactly sure... but looking down the road another 5 to 6 years... I can tell you this much...
* RWD (AWD a possibility)
* Good HP / Weight ratio (equal to WRX-STi or better)
* Smog Exempt ( Tired of dealing with CA Smog Nazis)
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So if we're looking at 2010, minus 29 years for smog exempt status, we're up to model year 1981. Here are some potential candidates...

The Europeans:
1988 Audi Quattro
1988 BMW E30 M3
1978 BMW 635 CSi
1980 DeLorean DMC-12
1954 FACEL Vega HK 500
1982 Ferrari Mondial QV
1985 Mercedes-Benz 560 SEC
1980 Jaguar XJ-S V12 (or SBC conversion)
1980 Jaguar XJ6 (SBC conversion)
1985 Peugeot 505 Turbo (built motor obviously)
1986 Peugeot 405 Mi16
1981 Porsche 911 Turbo (930 Model)
1980 Porsche 928 S4
1979 Renault Le Car (just kidding; wanted to see if you were awake)

And from across the Pacific:
1974 Datsun 510 (SR20DET conversion likely)
1985 Nissan 300ZX Turbo
1984 Mitsubishi Starion ESi
1986 AE88 / AE86 Sprinter / Trueno

Plus whatever I stumble on that qualifies for the +25 year DOT exemption.

So yeah, I've been giving it some thought...
good thing I hang on to my cars for a long time..