Here's an idea for a new challenge mode:
~No buildings (Sugar lumps still grow luckily, but you obviously can't spend them)
~Seasons are free
~Permanent upgrade slots don't transfer (Except for Omelette, any Golden Cookie upgrades, and Bingo/Research Facility)
Achievements Only available in "egg" mode:
~Bake 1,000 cookies in this mode
~Bake 1,000,000 cookies
~Bake 1,000,000,000 cookies
~Purchase a kitten
~Reach 21 Billion CpS (The CpS of a Chancemaker)
~Embryo Complex: Name yourself "egg"
~Impulse Purchase: Buy all the useless upgrades possible(i.e., all the upgrades that affect only buildings)
~Ascend having earned at least 1 Heavenly Chip
I'm sure there are a ton more creative achievement ideas that I'm just not coming up with, so feel free to leave some ideas below, and let me know what you think!