No point in making a suggestion any more it'll be burried so deep nobody will ever see it. That said I have more questions than actual suggestions. I say questions because I don't know if any of these are actually complex engines.
What about an electric engine? Does it count as an engine? Are they simple?
What about the new Porsche 918 Hybrid engine? Electric power and gas power? Are they technically two engines?
What about the Audi E-Tron Quattro Turbo diesels in their LMP1 cars?
What about a Formula 1 engine that revs to 19,000 RPMs?
Are any of those complex engines?