On MidMazar's point, I really think that if you made a few resin engine sets for the '98-current rally cars (Pug, Focus, Fiesta, Corolla (easy enough using the GT-Four engine from the T kit), etc.), these would be MASSIVELY popular. I would buy several to use for some serious detailed kits. They wouldn't be too very difficult vs doing an entire car.
In that same vein, I would love some aftermarket parts to use for trunk detailing rally cars. Just some things that might be able to be used on several cars, like fuel cells, etc. Just a thought.
Love your stuff! Keep it coming!
Daniel