Servant of Rigidel - After your first 10 of each building, you may only buy building in groups of 10 or 100.
A few notes about this challenge mode before i get to the achievements that it could offer. First off, this challenge mode would have to have Born Again restrictions in addition to the requirements listed above because otherwise progression would be absurdly fast and any challenge specific achievement would be obtained pretty much instantly.
I thought of a few achievements that could plausibly be a part of this Challenge Mode (sorry that I'm not creative enough to come up with names for these achievements):
Have 10 mines (and 10 of every previous building) - Reaching 10 mines seems pretty reasonable as a regular achievement to me. I was able to get to 10 mines in about 14 minutes, with my only golden cookie being a Frenzy at the 8 minute mark, so it seems pretty achievable. Without the frenzy, I'd imagine it would take more like 20 minutes, which is still not bad at all.
Have 10 temples (and 10 of every previous building) - I mainly selected temples because I feel like it just makes sense to have an achievement for reaching 10 temples, seeing that Rigidel is a Pantheon spirit. With that said, while I have not tested this, it does not feel too unreasonable. It would probably take a few hours of play.
Have 10 of every building - This one I could see either being left out or made a shadow achievement. Having tested a little bit with this restriction, I found that progression slows much faster than it does regularly because you can purchase less efficiently, and because of this this achievement could easily end up taking days to complete. I know that Orteil has no problem making achievements which will take absurd amounts of time to complete (like getting all the level 10 building achievements or endless cycle) but it might not be a good idea simply because it could end up being tedious and unfun.
In addition to these achievements, I feel like progressing to a certain point (maybe after getting the 10 of everything achievement?) there could be an additional reward, such as Rigidel being twice as effective or reducing the amount of time it takes for sugar lumps to mature.
Thoughts/ways in which I could improve this idea?