So, usually when I make suggestions I try to keep them as tiny as possible because I feel like the game is polished enough in terms of content, and Orteil already does an amazing job at updating the game. But one feature that I always felt was a bit underwhelming and unpolished where the challenge modes. I mean, there is only one so far, and it's basically here to make some of the OG achievements (hardcore, true neverlick...) obtainable again after the legacy update. We had a discussion the other day on the slack group about hard strategy-based achievements such as speed backing, true neveclick or four-leaf cookie (really awesome achievement btw), and I feel like it never hurts to have more of those to break up from the "let's wait a few days to get enough resources" ones, which are an overwhelming majority.
My suggestion is to have a new challenge mode based on seasons (which are a really good mechanic to base a challenge on). It would be like a "season chaos" mode, where you have no control over the seasons anymore, but they will change on their own, probably every hour or half-hour. The gimmick would be that you don't have to have the season switcher, or spend cookies on switching season in order to get all benefits from all seasons, but at the same time they will change on their own (maybe through a set rotation but i feel like it'll be better to have them being random). An achievement tied to this mode would be to just get all drops from all seasons (all 20 eggs, all 7 halloween cookies, all 7 christmas cookies and all 6 hearts). This would add a strategy element, because you would have to stay alert for deers during christmas to get as much as possible before it changes, and you could decide to kill all wrinklers during easter or instead choose to save them in case you get halloween or something. To make the challenge harder, maybe this mode could prevent the effects of the starspawn/starsnow/starterror/startrade/starlove heavenly upgrades and/or the effects of the spooky cookies/let it snow/ hide & seek champion achievements.
While I'm on the topic of strategy-based achievements, karl suggested an achievement about clicking X amount of golden cookies in a row, without missing one. Basically just stare at your computer and don't miss any of them for like, 27 or 77 cookies? Another idea would be to click X amount of golden cookies from a cookie storm. Once again, it's all about being good at clicking stuffs, a bit like a QTE.