Quarian/Geth as 4th faction.

Why would you want to fight the geth? Do you want to boss to be a mini-repear, built by the geth and controlled by a genius quarian that they had on their ark? Do you not remember tactical scan and arc grenades??? Fight? The?

Because that's the only story-branch that can supply the full new enemy faction w/o a huge DLC with a completely new species. 3 enemy factions is a bit repetitive.

Yeah, well, I am immune to the mesmer of the swaying hips, so I did not think of it.

By some reason they got mixed in with the Outlaws, no?

I could definitely see the Geth returning as a new enemy faction that would be sweet. But honestly Geth make their own future, they certainly don't need or want us or the Quarians meddling in their affairs. Although it would take some explaining why they would bother to assault our colonies unless they perhaps need to harvest the golden worlds for resources to continue on their journey or like us to continue the construction of their megastructure.

The worlds with the best resources are predominantly inhabited by outlaws so it wouldn't be hard to imagine that we would be the ones to initiate hostilities if they show up on our planets and we end up starting another war with them through our intolerance and fear.

Edit: Also it would make sense that the Geth would be the first to evacuate the Milky Way galaxy given they were the first species to encounter Sovereign and indoctrination in the flesh so to speak.

As long as headshots make their visor shatter and they ooze out of their suits like elephant toothpaste.

This game doesn't need to ruin them too. I bet that female ass would be flat if they put them in Andromeda definitely not something you could slap and watch the wave go from one side to the other and you know for damn sure they wouldn't do the Geth justice.

The geth would make sense lore-wise, there are plenty of heretics left after ME2 (and even before the Reapers took control in ME3), and they could've somehow hitched a ride or followed the Quarian ark to Andromeda.
It'd be cool to fight against a Colossus. Just no more stagger-drones and Lolturrets.

And where is the money and manpower for this faction going to come from again?

Well, maybe they can put a few Easter eggs in the mysterious Benefactor's Secret Organization that's likely all humans... which, i suppose would mean no DLC...