Instead of modifying the pops themselves, perhaps an option to instead convert planets to "AI worlds". AI worlds cannot produce raw resources, but have infinite pop capacity and amenitities and ecunopulis like distrincts for productions. Perhaps with a special habitability required for these worlds.

They transcend the physical form, and send any pops on these worlds into an entire universe of their own making. Go the way (hopefully not completely...) of one of the precursor races.

You have an option to become the contingency and go full determined exterminator mode(with new endgame technologies, buildings, etc. and a bunch of events dealing with the Contingency)

I'd like the idea (for the synthetic ascension) to choose from like 3 different paths:
- one could be the gestalt project you already implemented
- one could be virtualization of pops, a decision that can be actuated on a planet to transform the entire population into digital programs, pop growth alts, pops on a digital world get a special trait after a month that prevents them from resettling, enourmous bonusses to research and unity, digital worlds have no mineral / food / alloy / strategic resource production possible
- another one could be Synthetic technospheres, abandoning the concept of living in planets the entire population is housed in enourmous fleets of technospheres that produce resources and provide fleet power (it could be done by perhaps using an event chain that costs a lot of minerals / alloys and at the end for each planet of the empire spawns a technosphere fleet proportional to the empire population). Multiple Juggernauts are spawned and become the shipyards of the technosphere fleet, your stations automatically become tier 1 outposts and you cannot upgrade them anymore, planets you once owned become automatic syntetic factories that produce 1 small fleet of armies once every 3 months (90 days)

for the last one idk how to circumvent the problem that an empire needs at least one planet or it's considered defeated, perhaps there are ways to go around that. Also the last one requires special ships that have negative fleet upkeep or something equivalent for generating resources and also could have a special casus belli against them (end threat?) because you'd basically become an endgame crysis, or perhaps like the infinity stones mod everyone declares war on you after a while.

I'm a fan of the idea of synthetic virtualisation. I think there'd be two aspects to it: pops and leaders.

Pops would receive the "Virtual" trait: "These synthetics have the ability to easily transfer their minds from their physical bodies to other systems." By itself, it would only substantially reduce housing usage (most of their needs can be taken care of in the virtual world, but they still need to do some things in physical reality, because the normal districts and buildings are designed for physical workers, like organics).

There would also be special buildings that provide a large number of special "virtual" jobs. These could only be taken by pops with the Virtual trait and would only be for things like unity and research, not physical resources like alloys and consumer goods. Their advantages would be slightly greater output compared to normal jobs and, more importantly, a complete reduction of the pop's housing usage to 0 (because now, there's no reason to exist in physical space anymore). As suggested by previous posters, there might even be specialised "computer planets", where only "server districts" allowing these virtual jobs might be available.

Egalitarians might also have the ability to set "simulated paradise" living standards, which works like Utopian Abundance, except only for Virtual pops and reducing housing usage for unemployed pops in the same ways as the proposed virtual jobs.

For leaders, I'd love to see a backup mechanic. Once your mind is virtual and easily transferred between different systems, you can just create a backup copy that can be reloaded if your physical body is destroyed. It'd be great if virtual leaders could simply "respawn" (assuming the cost of building a single body is negligible) after dying, regardless of whether it was due accidental death, some event killing them, or even death in battle. Basically, rather than just never dying from old age, they'd be truly immortal (as long as your empire and its backup servers survive).

It might also be interesting to introduce the concept of artificial superintelligences. Think the Minds from the Culture books. Basically, you might construct a megastructure (a small one, on the scale of a gateway or habitat, not like a Dyson sphere) that grants you one extremely powerful leader. You could, of course, construct multiple of these for multiple such leaders. You could even introduce extremely powerful, but accordingly expensive, new ship computers.

Implementing all of these might be too involved or overpowered compared to the other paths, I wouldn't know. But hopefully, one of these thoughts might be interesting in some form.

Thank you very much for your ideas and for time that you spent to write these enormous texts!
I love these ideas :3
If my skills in making mods are good enough to bring these ideas to life, I'll do this :3

I think it's important to keep in mind what kind of (distinct) playstyles you want to create with those implementations, and if those lead to noticeably different effects.
For virtualisation, would it really make a practical game difference compared to regular Machine intelligences, synthetic ascension or Gestalt (psionic) ascension?

The biggest issue with virtualisation is: how would you implement the distinction between the programs and the physical machines used for work? And how would you consider pop growth?

For example: how would you implement resettlement with purely virtual pops, and the husks they sometimes use as bodies? It creates a lot of issues on the gameplay-side. The same goes for upkeep: in synthetically evolved species, upkeep breaks down into alloys and energy per pop. Wouldn't virtualisation create a messy situation to code by uncoupling those?

You could try to play that only the physical jobs require alloy upkeep, but then you'd have to code to transfer the alloy upkeep from a simple value per pop to every. single. job. affected. Not impossible, but a harsh task to implement and easy to miss a few things, especially if you want to avoid compatibility issues with other mods.
If you ARE crazy/determined enough to do so, then you could implement unique buildings/megastructures making use of this virtual existence. it would also solve your resettlement-issues.

For example, you could create 'server'-buildings, holding a lot of jobs and housing, that reward an incremental boost per pop to research fields and admin produced on said planet (e.g. 1% per pop, capping at 20). This would require that virtually evolved pops require no Consumer Goods-upkeep, bc it would be a bit absurd if they did. They're on a server. Who needs coffee mugs when they don't have caffeine receptors? (Plus it would create a massive demand for goods, breaking players' economies...)
With these two,

The 'backup'-suggestion seems quite compelling. Since synthetically ascended pops are immortal besides the rare crash, having an opportunity to 'download a backup' of them would be interesting. If you can cause a temporary event (in Situation Log) to fire once a leader dies (eg. 1-2 months of time to act, use of engineering research) to gain it back, just with a reduced level (e.g. -2 or half), you could create an interesting choice to roll for a better leader or stick with a diminished version of the defunct one.

Some of the suggestions regarding superintelligence-megastructures are things I saw suggested on the Gigastructural Engineering-mod discussion thread. They might work better as part of that mod rather than this particular Gestalt-mod. (They already have a Matrioshka brain and Planetary Supercomputers, so...)
If you would implement them somehow, then superintelligences could be flavoured with being hacked through the Contingency, causing 'revolts' of things they commandeer. Having a superintelligence leading a massive fleet turn on you because your firewall is still useless could create...interesting scenarios. Or stability plummetting in an intelligence-controlled sector....

@PiFi, I had some ideas about virtualizatiton. I'll see what I can do with this.
The "Backup" feature feels great. To make a district or a building called "Backup server" and when a leader dies, there will be a chance that you can restore him from backup (and also there will be a change that the backup files were corrupted).

About Superintelligence...I totally agree that It would have a good part in Gigastructural Engeneering-mod rather than in mine.

Thank you all for you such long comments :3

I'd suggest looking at Eclipse Phase for ideas; for instance Eclipse Phase has "Infomorphs" which are sapient intellects that are kept in a purely digital form, capable of doing any work that does not require physical manipulation; and they have the ability to upload themselves to a synthetic shell as well as making backups of their personalities.

A transcendence like the computer in the short story "The Last Question" by Isaac Asimov. In that story, the world built a computer called VAC, which was asked the question of how to reverse entropy, and began collecting data. When it become obsolete, VAC designed new versions of itself using more advanced technology, and eventually transferred itself first partly and then fully into hyperspace, taking advantage of the hyperspatial physics and omnipresence afforded from being an energy construct to gather more data.

So perhaps something like this? Your synths begin to upload themselves into hyperspace, becoming entities of superluminal thought and endless potential.

Another idea you could do would be to be able to create duplicates of existing commanders skillsets, as you're copying data into a new command unit. That along side with their leaders being unkillable due to backups could be useful.