After reading something similar to what dpareja said in another thread I starting thinking a bit and was getting really intrigued by this.
The idea is that you have a form of 'Hardcore' mode (nothing to do with the achievement Hardcore). Where you cannot save the game and the progression is only stored if you ascended (so with at least +1 prestige point and pressing the legacy button).
The idea is that you start completely new game (except keep the achievements you can get from Born Again mode, like Neverclick etc.). If you close the game before ascending you will lose all progress, so next time you open the game you will be back at the ascending screen as if you just pressed reincarnate last game. (So in other words you get all your heavenly chips and need to choose upgrades again as if you had just ascended in the previous game). I thought the beginning of the ascension was best, because it allows for a possible different tactic by buying different upgrades with heavenly cookies before trying again. And maybe you don't want to play that mode anymore after losing all that progress, so you can just switch back to normal mode immediately if you want. Also, as far as I know it would create a new seed for the next game you would try (not 100% sure on this one).
So basically this adds a complete 'new' game mode separate of the original game. Unfortunately I don't know anything of coding so I'm not sure how technically viable all this is. But to me it sounded like a very interesting new way of approaching the game.
Now because I always overthink things I also thought of these possible variations of it:
The 'Angels and Demons' upgrade tree will become utterly useless in this mode and because sometimes it's not bad to 'pause' the game so you can turn off your pc and return at a later time where you left off, I came up with these things. Again, I have no idea about how technically viable it would be and frankly I'd already be happy with the other implementation, but here it goes. I left the synergy buildings and kitten upgrade untouched. I tried to keep the idea of the tree similar to what it is now.
Note: I'm bad at naming things so I called the mode 'True Hardcore' and you may just ignore the other names if you so please.
[EDIT: So it seems I can't post links to my excel pictures, so I'm gonna try and do it here, hopefully not make it look too ugly]
First upgrade (Forbidden Rituals tree) to unlock the rest of the 'angels and demons' tree:
You can choose to return (descend) to your previous gamestate, as it was before ascending, if done so within one hour.
After one hour has passed after ascending you can only reincarnate into a new game.
When you return/descend you gain cookies as if your game was running with 5% CpS.
You can return/descend once per tier that is unlocked within the Forbidden Ritual tree.
(This is the first tier, the second one is unlocked by having both next upgrades unlocked, one on each side)
!!!WARNING: When returning/descending you have the same prestige and heavenly upgrades unlocked as before you ascended. And if progress is lost (game closed while ingame) the game will 'reset' back to before you first reincarnated(!!!).
Upgrade for tier 2 till 8 on the 'light side':
You gain another +10% of your regular CpS when returning, for a total of 15%.
You gain another +10% of your regular CpS when returning, for a total of 25%.
You gain another +10% of your regular CpS when returning, for a total of 35%.
You gain another +10% of your regular CpS when returning, for a total of 45%.
You gain another +10% of your regular CpS when returning, for a total of 55%.
You gain another +10% of your regular CpS when returning, for a total of 65%
You gain another +10% of your regular CpS when returning, for a total of 75%.
Upgrade for tier 2 till 8 on the 'dark side'
You are able to return within 2 hours
You are able to return within 4 hours
You are able to return within 8 hours
You are able to return within 16 hours
You are able to return within 1 day and 8 hours
You are able to return within 2 days and 16 hours
You are able to return within 5 days and 8 hours
Upgrade (Power of C'thulu) after all tiers up till 8 are bought (tier 9):
Synergy upgrades are 2% cheaper.
You gain another +5% of your regular CpS when returning.
You are able to return to the game within 2 more days.
New upgrade (Power of Yog-Sothoth) (tier 10):
There is no time limit anymore for when you choose to return to the game.
(Note: There is still a limit of the number of times you can do so.)
Beyond 7 days and 8 hours the rate of your CpS, that you gain when returning, goes down to 0.5%
Final upgrade (Power of Azathoth):
Gain another 10 tiers, making you able to return to the same game a total of 20 times.
Maybe a complete change would be better or just some filler upgrades for the sake of the mode, like I said this is just something secondary I came up with.
The idea was that if you have the 'True Hardcore' mode selected and are able to return to the previous game, because you bought the upgrades. Than after pressing Reincarnate it will give you the option to Reincarnate or 'descend'. Also I'd suggest putting up warning boxes for selecting the descend option and just the True Hardcore mode in general, because you know. People will get angry for losing progress when they do.
To add to all this here's an idea for possible achievements:
You made it!:
Ascend one time in True Hardcore mode
There's no stopping now:
Ascend 50 times in True Hardcore mode
I'll be back!:
Return/descend one time in True Hardcore mode (use the return feature to go back to a previous gamestate while in ascension)
Return/descend 10 times to the same game in True Hardcore mode
How long can this go on:
Have at least 500 prestige in True Hardcore mode
I'm not sure if it's a good thing to have very 'lategame' achievements for this mode or not. But I figured I'd just give some ideas.
I hope I was able to be clear about the ideas I had and hope even more that it can somehow inspire for new gamemodes in the future.