Needs a million upvotes. It costs a large pile of money to design an all-new engine from the ground up that meets modern emissions and is reliable is service for hundreds of thousands of miles. You should be thinking billions of dollars. They only way to make this affordable is to amortize that cost over many many cars of many many years. Sure you will tweak the design here and there for various reasons, but the initial investment to get the first car to a customer is too high to dare start again anytime soon.

Of course with electric cars obviously coming it is questionable if any engine is worth starting today. There isn't much worth doing that a minor tweak to an existing design cannot do - and even if there is something it is questionable if engines will be sold long enough to be worth it. (at least for cars - if you target boats or construction equipment or such maybe - though you will note smaller boats typically just take an existing engine and tweak it because there are not enough small boats sold to be worth a new design)