For a first post this may be a bit brave but why does there have to be one engine type. One of the great things about group B was the oppotunity to use different engines.
Just pulling this out of the air but, use the chassis/transmission of the S2000 concept. Allow N/A engines of a capacity up to 3L, turbo cars up to 2L & if you want to go completly mad supercharged & turbochaged/twin turbo cars up to 1.6L. I know that the engine regs would have to be written very carefully but it would certainly be interesting. BMW might join in with one of their I6 engines, suzuki could use their S1600 engine & strap a couple of turbo's on & mitsubishi/subaru can just develop their evo/sti engines. It means all manufacturer's can use an engine from their own stables & that has to be a plus for marketing.