Personally I really like grilling before a big feature, it's a Matt Pocock skill that will iterate and ask you a lot of questions so you REALLY know what you are building. It's particularly good for complex and big changes, so you actually know what the AI is gonna build before it does. Of course, that's not always the case, sometimes you don't care about the specifics. But I think an integrated "plan mode" is really not needed, maybe a "brainstorming" skill would be more useful.