This is more of a thought experiment than anything else at the moment, but wouldn’t it be a fun(and probably very difficult) scenario if you were to play as one of the people who ended up stranded on an alien world?
I don’t think the portals are one-way tears in space(currently they’re unstable spacial anomolies but they were once used for travel), so in my mind it would be entirely reasonable for someone to have gone through a portal on the day of the cataclysm and ended up lost on the other side due to it closing or them being blocked from going back by some sort of monster. It would be hard to survive there, and you’d probably constantly need to produce a breathable medium of air or slowly die of the poisonous atmosphere. Most people’s first tries would also fail misserably due to them not knowing what is safe to eat and what not, and to think of all the horrible new monstrocities you’ll have to survive.
I know it’ll need an entire new set of tiles, flora, fauna, etc to make it work but it’ll be a fun alternative for most people i believe.