First and foremost - open choice of chassis and open choice of engine. After that, I'm largely unconcerned...

Tempting to say a big V8 or something similar, but I suspect a small-capacity turbo will be used to make it more relevant to F1. Ideally then, similar spec to F1 but with power capped - maybe even allow F1 engines with restrictions. And restrict aerodynamics as far as is practical - I wouldn't allow a DRS system, but again I fear one may be included because of "F1 relevance".