Lack of sentient species?

They called MEA a saga, so unlike the ME trilogy, this game will keep expanding with each new story added. Sort of like DA and without a repeat protagonist most likely.

Javik did indicate that when the Protheans ruled the Milky Way Galaxy, all the species "eventually called themselves Prothean." So, perhaps that Kett are similarly dominant and the Angarans are resisting assimilation by the Kett and the Remnants are the remnants of the other species that have already been assimulated by the Kett.

It's one star cluster. It would be very weird to have 10 Andromeda inteligent species there.

Also Andromeda is a lot bigger than the Mily Way and as you said there are no Mass Relays there, so we should expect their civilizations to be less widespread than the Citadel Races.

That being said i still hope we find some civilization with less advanced technology.

Not really.  Diversity flourished in the Milky Way in the current cycle because the Asari were driven towards a cooperative government.  A domineering government (like the Protheans) eventually sucks all the diversity out of the cultures it absorbs... it's got nothing to do with Reaper/synthetic/transhumanism.  It would, in fact, be a logical theme associated with colonization.  The newcomers upset the apple-cart of the dominant group simply by introducing completely new and completely foreign cultural diversity into the galaxy... new ideas, new weapons, new ways of dressing, new ways of thinking.

It still doesn't preclude diversity still existing... just going to perhaps take us some time in this game to find it.  It also doesn't preclude Bioware adding diversity into future games.

I'm saying it could easily have a story-driven purpose... or were you really just fishing to hear more complaints about Bioware being inexplicably lazy?

Indeed, earlier this month GameInformer posted that there will be more advanced species yet to be revealed.

Also those raiders we seen so far in trailers?

They could be a new species that only appears human due to them being under full suit of armour and helmet. It was the case for Batarians in Blue Suns and Asari in The Eclipse merc groups in ME2, as normal grunts were all wearing helmet.

It's also quite unrealistic to have so many human raiders by the time ME:A starts, so there's that too.

They could be. I'm pretty sure "raider" is a generic baddy class type, bandit or mercenary.

If I remember right, the story starts two years after the AI first gets to Andromeda because our ship gets delayed or something, that's at least a little time for deserters to start factions. We already saw that one black woman (forget if she had a name) with krogan bodyguards that Sara talks to in one of the trailers as being a mercenary type.

That's going off topic a little though. Given their body shape, Raiders could be humans, asari, and maybe turian, or maybe some new alien we don't know of yet.

And yes theres plenty of resons to have more species than just explain it away with "its only one cluster". Maybe its a main trading cluster for multiple species, or its a pioneering paradise, many races prospecting/competing for minerals. I could for example add 3 more races to the cluster:

1 coming from the other side of the cluster to the kett, pure greed and expansion in mind.

1 pre spaceflight civilization on the edge, that maybe gets first contact with us, or into some disaster with the other races.

1 more that is sapient, but so alien we struggle to classify it as one, ala Horta of Star Trek.

The fact that it's easy to write species into the narrative doesn't mean that such would fit the narrative. Clearly, this particular story has a tighter focus, revolving around the Heleus Cluster natives (or at least residents) and the Milky Way interlopers. Assuming we're fortunate enough to get additional games, it's safe to assume more new species will appear.

Valid point.

Other possible explanations apart from what people already wrote (ie we are limited to only a small section of the andromeda galaxy) are :

-1 The milky way was basically an unified galaxy with a peacefull central goverment when we are first intrdocued to the ME universe. Peace and amicably relationship encourage travel and mixing of species and culture so this was definitely a good reason for having many species intermingle in the citadel on the other hand if andromeda is home to a various sets of government at war with each other or simply not in the most cordial term with each other the chance of finding many different races in one place drop dramatically

2- A more technical reason is that the guys from bioware had to recrate in the new engine all the graphical assets of krogan,human,turian,salarian and any other old race that will show up in andromeda over creating the 2 new race so personally i'm not surprised by seeing only 2 new race (for now for what we know)

It's a single star cluster as opposed to the entire milky way galaxy so 3 new species for this region for now is about right imo. The setting and gameplay mechanics are arguably far more nuanced now so there's arguably a lot more to re-establish this time around for veterans and newcomers alike. There aren't quite as many species introductions as ME1 but all of these are new to newcomers:

Asari
Salarians
Turians
Krogans
Possible minor races
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Angarans
Khet
Remnant
Fiends & other wildlife