But rally is the ideal tool to promote your technology. Cars are also stil recognized as the production models. So thats an advantage for wrc over championships like f1 or even Dakar.

Use different drivetrains, but let them produce the same power output! That is a start.
For aero and suspension etc. Rules are still the same for all.

Constructor advertising a hydrogen powered (lets say) Toyota Yaris, can make posters/advertising whit car that is for sale in there showrooms! Thats 1 on 1 proof of how good the technology works. Customer (not die hard rally fan) doesn't need to know that (let's say) Hyundai whit petrol/hybrid is maybe faster sometimes.

To cut costs is also difficult, no space frame cars seems like a good start, R5 shell we could say. There's a lot of decent race teams around that can build cars like that(hansen in rx for example) those can than buy power unit from manufacturer like millner suggested.