Why Geth and Quarians won't be in Andromeda and why you should be ok with it.

Except that the Rachni, as of the time the expedition leaves, are not 'friendlies'. I cannot imagine any scenario where they would be invited along on the trip. Same for the Geth.

Dude, the rachni don't create something from nothing. I get you like them but there's no need to overstate things. The rachni need resources to create ships and weapons just like everybody else. The only way they differ from the geth in terms of practical applications is that they can act on the physical world immediately whereas the geth would need to construct additional platforms first. And the geth (or krogan for that matter) aren't "dedicated to one task" any more than than any other species is- they all want to survive and advance. Also, there's no need to assume geth aboard the arks wouldn't have physical platforms at their disposal, even if their duties would primarily be in the ships' systems. Not bringing some platforms is foolish in case of catastrophic failure and the platforms are quite compact and can be stored anywhere with no resources being expended to that effect (say what you will about rachni and toxic environments but even they need an atmosphere). And constructing additional platforms never seems to be an issue. Remember the Alerai? A few geth pieces were able to assemble and construct a whole platoon of geth over the course of a few days.

No one's doubting the rachni's usefulness. But unlike the geth who can speak for themselves and approach the AI administration about joining, there is no one to speak for the rachni and convince them they're not a threat. And a 600 year long mission is no place to experiment with cloning and controlling creatures that never been anything but hostile as far as anybody knows. A geth can walk up to a human, make a gesture of peace and attempt communication. There are simply no rachni left to do the same thing and no one's going to risk bringing them back, especially on a trip like this. The only way is for the rachni themselves to approach, and there aren't any left to do that, short of a real contrived asspull. And no, going to Andromeda is not on the same level of contrived. A secret project to escape the Reapers makes perfect sense.

As for meta reasons, it doesn't matter whether the rachni make the trip or not if Bioware doesn't give a shit about actually giving us "alien" aliens. Precedent doesn't matter if that's not a priority for them. And hell, remember the geth? They wanted to be real boys in ME3. What if they do bring the rachni, except there's an equivalent change to their behavior? I get what you're saying but the rachni won't make or break this issue- it's already done. Have you seen the Khet (kett?). Humanoid rock monsters. I think this ship has sailed.

I haven’t read all of this thread, so sorry if I repeat stuff.

But I think for the geth there could be a very good reason why they are there and it might actually make for a pretty cool mission, too.

If the AI leaves in 2185, that’s after ME1. This means the schism within the Geth has already happened and both sides (geth and heretics) know the whole truth about the reapers at this time. They are also known to keep a close eye on the organic’s extranet, so they’d know about the Andromeda Initiative.

Because of the existential threat that the reapers represent, the geth (not the heretics) decide to infiltrate the program. Now, from all we know geth are a combination of software and hardware. While they can upload and download into geth platforms and servers, there seems to a restriction that prevents them from just interfacing with anything (otherwise, ME2’s Overlord DLC should already have happened as the heretics could have just taken over any machine connected to the extranet).
We know that AIs require a quantum bluebox hardware, so I guess that’s the constraint.

But the geth could program a VI virus that infiltrates some software in one of the AI ships. This VI would lie dormant until they reach Andromeda.

Now cut to Ryder in Andromeda. S/he gets a call that the Nexus lost contact to one of their manufacturing vessels. I’d assume they may have brought ships that are specialized on manufacuring spare parts and other needed items. Or maybe it’s just a special level on the Nexus/one of the Arks. Anyway, Ryder should investigate. After a bit of clue hunting, we find the ship dormant and when boarding it, some strange things are going on as the geth VI tries to take control and kill the crew (and us). Kind of a bit like that N7 mission in ME2 on the space station where the faulty VI tries to kill Shepard. At the same time, the VI tries to use the manufacturing rigs to create geth hardware (similar to how they did on the Rayya in ME2).
Ultimately, Ryder has to prevent the geth VI from spreading to other ships or even the Nexus. We succeed in that but the geth that are already created manage to take control of the ship they already have and take off into uncharted space.

I like this idea because it reminds me a little of the replicators in Stargate. Having this gnawing feeling that even if just one of them slips through your fingers, you potentially still will have a massive threat on your hands eventually.

Also, since the Andromeda people have never heard about Legion or the geth as we know them from ME2/3, they may eventually develop a very different and unique perspective and/or relationship with them. I think there is potential there.

Nonsense. Having all the races doesn't make it a Mass Effect 4. If that was the case, having any of the old races or any of the old tech would make it Mass Effect 4. There is no line that determines that. Plus that is not even what Bioware meant when they said MEA is not ME4. They mean it is not a direct continuation of the story told in the Shepard Trilogy. 

If Krogans were able to join the party, I'm confident Quarians got in also.

Geth no. The departure happens between 2 & 3. Even if the Geth side with us, they're still being shot at up until the end of ME3. They aren't trusted at the time we head for Andromeda.

Fuck yeah Vivi best squaddie.

So you agree that having all the old races doesn't make it Mass Effect 4 then? 

Given that

1. The nexus includes species other than the council four, as proven by the Krogan
2. The Ai leaves before mass effect 3 and
3. Not all Quarians even choose to return from their pilgrimage

It is entirely possible that there may be a couple of Quarians in Andromeda. Geth on the other hand, not likely.

From the Game Infomer guy ama:

"Ah! So, here's the thing about other races: 4 specifically came to Andromeda on the Arks (Turian, Salarian, Human, Asari). However, the Nexus (which is kind of like Andromeda's version of the Citadel) was sent ahead of the Arks, and the Nexus had a more varied staff.

I don't know specifically which other races (apart from Krogan) got to Andromeda via the Nexus, but theoretically, it could be any of them."

So, as I said, if Bioware wants Quarians there, they'll be there. Question is: do they want to use resources in them?

What's a Quarian? Oh, them!

Already forgotten...

I agree. I wouldn't mind if one is a squadmate. If not, be a crewmember