How much code review you need is going to vary based on the kind of change. When my team has a large design decision, we anticipate that we're going to have to

How much code review you need is going to vary based on the kind of change. When my team has a large design decision, we anticipate that we're going to have to create some exploratory code that won't ever reach production. We review "spike" pull requests--maybe several generations, depending on the scope of the change. And if needed we have on and offline discussions about our alternative, the tradeoffs, etc. I think this works a lot better than trying to impose design reviews on every change. Most changes shouldn't require this kind of scrutiny; you don't want to slow down 95% of code reviews for the sake of the difficult 5%.