Just my thing is....why would anything from the Milky Way be all the way in Andromeda? I can understand a couple random Lost In Space style explorers like Gillian and the quarians, or a single human, but its an ENTIRE GALAXY AWAY. We haven't even seen a fraction of the milky way, we know most of it isn't colonized.
We don't know enough about the plot of the game yet, or even if the leak details are legitimate, to say for sure.
But some of the fan speculation is that an ark might have been intentionally sent to Andromeda during the Reaper War, as part of a Plan B in case the Reapers win. Basically in that scenario it is the equivalent of the Prothean project where they put people in cryosleep on Eden Prime, in that hope that they could outlast the Reapers and recolonize the Milky Way when they departed. The only difference here is that the Council's colonization project is giving up on the Milky Way and setting out for Andromeda, which given what was known about the Reapers, is arguably the better plan.
So why Andromeda? You're stretching suspension of disbelief pretty far to say a human is out there, or that any Milky Way tech or other species made their way out there.
Actually Andromeda could be easily reached by the Reapers, who at their maximum speed of 30 light years per day could reach it from the Milky Way in about 230 years. The Council species, whose ships top out at 12 light years per day, would spend over 500 in transit. Time is not so much of a barrier however given that cryosleep is a thing within the Mass Effect series. The real obstacles for the Council species would be the need to refuel and drive discharge, unlike the Reapers, who have reached a level of technological sophistication where neither is required.
If Bioware is going to send a ship to Andromeda a couple of ways to do it are to have the Council species reverse engineer Reaper technology that allows a ship to travel without refueling or drive discharge, or to have the ship arrive in Andromeda via a wormhole. One interesting fan theory is that a side effect of the Crucible would open up a wormhole, with some of the ships participating in the battle for Earth being accidentally transported and stranded in Andromeda.
Another possibility I just thought of while typing the above... is that maybe the Crucible borked the relay network when it fired, and ships fleeing through the Charon relay after Priority: Earth were being dumped in random locations on the relay network. That might explain why the Normandy ended up alone on what appeared to be an uncharted garden world, instead of some predetermined rallying point for the fleet. If the Reapers had also been visiting Andromeda and annihilating civilizations there, it is possible that it would have its own relay network and perhaps even a connection to the Milky Way network. You could then have some ships from the Sword fleet dumped unintentionally in Andromeda, and unable to return when the relay network crashes shortly afterward. (its implied that the network needs to be repaired in the epilogues)
This has to be a galaxy so far removed that the reapers weren't bothering it. Which means so far removed from the Milky Way that we shouldn't be seeing anything that makes us think "Mass effect".
Once the Reapers being stranded in dark space was retconned, they had the potential to reach Andromeda as well as the Milky Way. Their speed and the fact that they don't need drive discharge or refueling, and are effectively immortal, makes Andromeda fully within their reach. Of course an ability to reach Andromeda doesn't necessarily translate into a desire, so its impossible to say at this point whether they were active in Andromeda as well. That is entirely up to the writers and they have the option of going either way with it, without trampling on lore.
If the Reapers had been visiting Andromeda and destroying civilizations there as well, it is likely it would have its own relay network. They would have most likely rolled with a similar game plan as the one they used in the Milky Way. If the Reapers built a relay network there it is also probable that it would have its own version of a Citadel, attended by an Andromedan version of the Keepers.
Even if the Reapers didn't visit Andromeda it is possible that there could be something similar to the Milky Way relay network, constructed by something native to Andromeda. The Reapers, which have been around for a couple billion years at least, top out at 30 light years per day. Anything native to Andromeda that had been around as long or less, might not have been able to exceed that speed. The Milky Way is about 100,000 light years across. Recent estimates of Andromeda put it about 220,000 light years in diameter. Traversing either without the use of relays would be possible for the Reapers, but it might not be convenient for organic species. Any species that reached the technological sophistication of the Leviathans or the Reapers might have sought to construct something similar to the Milky Way's relay network, to greatly reduce transit times between explored/colonized systems.
/speculation