The geth in Mass Effect were arguably one of the most interesting interpretations of AI seen in science fiction. They were intimidating enemies because of their history with the quarians, the impossibility of communication with them, and their worship of the Reapers. Furthermore, that they had isolated themselves from the rest of the Mass Effect galaxy and were purely hostile to any organic made it impossible to reach out to them.
Mass Effect 2 introduced Legion, a geth platform with over a thousand programs enabling it to speak and thus communicate with Shepard. They told Shepard about geth society and culture, how they think, the struggles they have as a young species, and their true view of the Reapers, which they call the Old Machines. Legion is the only geth Shepard speaks to in Mass Effect, unlike other alien species where there are at least one other member. Their perspective is invaluable and gives a massive amount of depth to the Reapers, the conflict about organics and synthetics, their philosophy, and the geth-quarian conflict.
The Issue of Legion in Mass Effect 2
Legion appears very late in Mass Effect 2 during the IFF mission, and there's only just enough time to complete his loyalty mission before the end game mission is triggered. This has been criticized as it cuts off a lot of his content, even though he's such an interesting and important character. This was a decision made late in development, and early cut Mass Effect 2 content shows that Legion's role was going to be very different, as a "Frankenstein" geth that Shepard assembled from parts found across the galaxy.
Perhaps this design lived on in Legion's hole and his use of Shepard's armor, but overall this idea would have diminished the representation of the geth in Mass Effect as Shepard would have control over the final product of their geth experiment. This could have mirrored Shepard's death and rebuilding by Cerberus, and Miranda's consideration of using a control chip to guarantee Shepard's loyalty. However, much like how the Illusive Man chose not to do that because it would diminish Shepard, BioWare chose to represent the geth as they truly were through Legion.
The Geth Could Return For Mass Effect 4
While the geth Frankenstein idea couldn't work in Mass Effect 2, it does have potential for Mass Effect 4, especially as the Mass Effect 4 trailer seems to imply the Destroy ending from Mass Effect 3 is canon. While the Destroy ending was one of the more popular choices because it was the only one where Shepard survived, it had the drawback of destroying most tech, including the relays and the geth. The destruction of the geth in particular was very hard to accept as it took a lot of effort to reconcile their war with the quarians and integrate them into the galaxy.
Mass Effect 4 doesn't have to go without the geth though, and the idea of a geth character rebuilt from parts has more potential for a post-apocalyptic Milky Way than Mass Effect 2 did. Rebuilding an individual geth from parts could ultimately become a way to restore the geth in Mass Effect 4. The geth squadmate could then go on to rebuild the rest of their kind with the help of the player character, or perhaps other quarian characters, and do so in a way that learns from the mistakes of the past. Rather than dictating what personality the geth has, it could develop how an organic mind would, as a true hivemind AI whose job it is to decide the future of their species without the threat of the Reapers.
Mass Effect 4 is in development.