Do you think there are more aliens hiding in the Heleus cluster?

Yes it would be cool of course.

Agreed. That's why I pointed out it was extremely unlikely that levo and dextro planets both existed in the same cluster. It might have made more sense to have the AI launch into at least two separate clusters just to make this work. Naturally, those clusters would not have been as large as Heleus.

How much of the Heleus cluster is unrevealed? Is there that much unrevealed to Harbor Another advanced Alien lifeform? I expect more aliens outside of Heleus but believe Heleus itself is mainly a Jaardan space.

I think that shows that cutting content can be a good thing, for I think the Oasis wouldn't have fit that well in with the story they seem to be trying to make for the Heleus Cluster. Now with that being said if they are able to have a good reason why we didn't find out about them in the first game and why they are there since it seems the cluster was being terraformed to something else, I wouldn't be apposed. I would rather that we find another race in another cluster though.

I agree.  There is so much left wide open at the end of ME:A that anything is possible.  Are we even sure the Jaardan is just one species.  Are they perhaps an empire like the Protheans.  We know the kett have exalted members from 1,000 or more species (according to the Archon - "I am the genetic inheritor of a thousand species.") and that they come from outside the cluster; but we don't know really how long they've been exalting species within the Heleus cluster nor whether they've managed to extinct the species they've exalted in the past.  The Angara have a history that they believed was true; but the reality they face now is that all that history beyond the time of the scourge was likely false (or created in their minds by the Jaardan).

So, yes, I believe there are other species in Heleus that Bioware hasn't revealed to us yet.  Of course, it will all depend on where they plan to take the story from here.

I'm sure. There's even plenty of undiscovered species hiding here on Earth.

I never even thought about this. If the Jardaan created the cluster it might explain why it's so large.

The problems with that conclusion is that there's no particular reason to think that we see all the systems in a cluster on any MET map, and there's no plausible action mechanism to spread dextro or levo life across a cluster. (FTL starship seeding could do it, but then you wouldn't be limited to clusters since everybody discovers the relay network.)

I much prefer finding more races in neighboring clusters.

Could be but then we're left wondering why Aya was unaffected.

For the main question, I want other races and actually more developed societies. Whether that's in this cluster or due to us expanding to the rest of the galaxy I'm not too concerned with. While they would not have to build relays to explore the rest of the galaxy without them travel time probably becomes part of the story and I'm not sure that is a good idea.30 light years a day might get you around a cluster in a short enough time period where we can ignore it, but if it takes months or years of time for a journey it becomes harder to ignore.

Just a few comments

1) I'm 90% sure that Oasis was the prototype name for Aya, there are many similarities in the lines about Oasis with Aya, both are hidden worlds that the Kett haven't found, etc.
2) The angara are not the only species in Heleus, just the only one we know to have survived the Scourge. H-019 had a primitive intelligent species who perished when the Scourge hit, and there is a torpedo found on Pas-40a that is potentially pre-Jaardan, so there is evidence of other aliens.
3) 1 race to a cluster is disproved in Mass Effect 1-3 because there are dead garden worlds in the same clusters as homeworlds. Not only that, but from a scientific point of view, it's a bit silly to think that a cluster can only have one intelligent species, and it is silly to think that one cluster must have only dextro species and another a different type. If Bioware wanted, they could have hundreds of stars in a cluster and a number of intelligent species, who could all be at different levels of civilisation; some might be at the space age, others at the bronze age, etc.
4) Although probably not native to Heleus, there are references to the Eealen and Thusali both ingame and in the cut script files. It's highly likely they were intended to be added at some point in a DLC or were cut because of resources/time.
5) There are also references to "ambient species" in the script files, but these are well after the Angara, Kett (Khet at the time), Eealen and Thusali were mentioned, which makes it possible they had intended on another intelligent alien species.

So do I think there are more hiding? Yup! ^^