I want to know more about the Jordan and why they created the Angara. I want to know more about the Kett's motives across the Andromeda Galaxy. Archon went rogue and I want to see their true motives. We are literally in one cluster. Think of it as the Local Cluster in the OT. There's so much to be explored on. Who's the Collector, Jain Garson's death, more species throughout the galaxy, introducing the galaxy to Relay technology if they don't have access to it already. Andromeda is a big place, and they've barely scratched the surface.
The Milky Way galaxy has been explored and done and has been fleshed out already. Maybe after they truly wrap up the Andromeda story, they can merge the two galaxies so that people can travel across them freely or something. It's been 600 years, Shepards dead and there are new challenges in Andromeda.