WARNING TAU CETI SPOILERS!!!!
So far in tau ceti we have discovered that the ancient are an artificial intelligence similar to the idea on the QMs that i spoke about and sirius was acted as a jail to the ancients as like Lagrange was to the QMs. So i think definitely the story after part 4 isn't going to work well if it sticks to how it is. So far are the end of part 3, the ancients have told us that they have located the mantis homeworld and that the plan is to launch an attack and potentially end the mantis race. By what we have seen through the mantis' capabilities they appear to be more advanced the ancients and that we've underestimated their power. As we went up systems their security increases and so does their power. Up to the point where if we aren't careful to the slightest they could butcher us. A hint shows the ancients used to be in all systems but got eradicated by the mantis and the remaining jailed in sirius. So if the story has gone like that then i might have another idea on how it could go after part 4.
Who are the QMs?
To start off, the signs the mantis are very advanced is that they have the abilities to mind control as they have demonstrated in part 2 where they took control of the ancients when we thought they were "allied". In the mantis homeworld there could very likely be experiments of new technology and completely new powers happening that they might be intending to use against their enemies. If we successfully destroy their weak spot on their homeworld then they would either all die or become powerless. At the end we conquer them and we discover experimental equipment they've been testing. One very noticeable technology is a large device that can mess with the universe and time. There are theories that suggest there are an infinite number of universes, universes with the same people but different outcomes. Since humans and ancients have only discovered that device we have no clue of its capabilities. As a result, we takeover into analysing their device and something goes wrong. We end up mistakenly programming it into releasing ridiculous amounts of energy to merge another universe into this universe. But since matter and energy can't be created or destroyed they can only be converted to one form or another. The other universe involved the ancients constantly evolving to the level where they become who we refer as the quadremetrics. They surpass their creators but don't just push them back as like the ancients did to us but exterminate them. When the universe merges, many things start to happen, Since all of that would be too much on the expansion of our universe so far, many things get replaced, the ancients on our universe will be replaced as the QMs and are evil and against us. since the mantis are dead, this means we are able to replace their dead matter and stay alive, sirius singularity comepletly becomes a mess and becomes the "Lagrange singularity" and contains gates all around it that lead to many star systems.
The other universe that merges to ours is older than our universe unnoticeable by a million years, hence so much has happened with other stuff, the "sirius pirates" have begun invading other completely new alien civilisations using their stargates in lagrange to get to them although they aren't human pirates they are just another evolutionary sapient species that have been evolving sometime after the QM invasion, 20% of the time we will get transported into their systems and help the civilisations from invading pirates and ally with them. There are also other systems that get invaded by rogue methanoids, methanoids by what i have noticed have been secretly working with the ancients (in venar) and that they are left alive by them in that universe and are incredibly advanced. (although the QMs always monitor them to ensure they never surpass the QMs and become dominant) they also try to invade other alien civilisations in other systems you can protect and defend.
However, the main part of the story focuses on the QMs and you see their stargates nearly 80% of the time in their systems that transport you to many systems with dead ancient civilisations taken by them and at the higher end of lagrange you even get to a realm between the 3rd and 4th dimension and reach their hivemind.