The perfect Mass Effect game to me has a nice compact, well paced and developing story with a threatening, but not infallible villain like Saren was. Then you take Mass Effect 2's mission structure where every mission had its own self-contained story that not only helped flesh out party members, but showed you that the Galaxy was more than just Shepard, with billions of inhabitants leading their own lives and/or trying to destroy other's lives. Also ME2 had a bunch of hub worlds, and every mission took you to some place different. Hub worlds in my perfect ME game would be even larger than ME1 Citadel, with different tiers and buildings you can go to, reachable by a skycar you could either manually pilot to explore the giant galactic cities, or use for quick fast travel. Then you mix in ME3's smooth gameplay, weight system, and skill trees that made each build unique, different to ME1 which was riddled with passive boosts.
I love ME2 the most because it best displays the culture of the universe best. Much of ME1 is visiting barren planets with the same copy and paste space station. If not that then you were forced on a mission that didn't allow one to really explore the culture of the universe, save for the Citadel ofc. ME3 eliminated any kind of exploration that ME1 and 2 offered because of the plot. ME2 took you to so many places, and you got to meet so many people with their own personalities. Each recruitment/ loyalty mission also took you to different areas of certain hub worlds, like how Mordin's mission took you to Omega's apartments, or Samara's mission allowed you to enter a new section of Omega's clubs. I loved how everyone had their own story, and Shepard was allowed to be a part of it while never truly being the center of it until the main story missions. I also love how ME2 had side missions focused on the different merc groups, which showed that the galaxy is a living place which moves on, whether Shepard is there or not. ME2 for me made the Mass Effect Universe one that I wanted to actually live in. Going to Omega with all the awesome neon and looking out the windows into space, going on the actual Quarian flotilla... I really hope ME:N expands on all of that. With next gen technology we can go beyond being shoved in the small hub boxes that ME2 admittedly had, with static NPCs copy and pasted everywhere.
One of the biggest complaints about the trilogy is how choices don't matter. Why play Arrival and have Shepard kill 300,000 Batarians when you can just skip it, put the burden on someone else, and achieve the same results? Why help Liara become Shadow Broker when she achieves it anyway? Maybe because Shepard doesn't have to be at the center of every single galactic decision. People still have lives to live, things to do, and they'll do them with or without Shepard. Shepard might have made it easier, but S/he wasn't required. When they are required we get a Messiah "special snowflake" and everyone is so averse to that these days. I hope the new character avoids the messiah trope, and I hope the story of ME:N doesn't try and raise the stakes even further.