AU. No Reapers, no extinction cycles, only Protheans (Who disappeared for unexplained reasons prior to the game, and who did actually create the Citadel/Mass Relays this time). The universe would have the exact same history as the first, up until the Normandy mission to Eden Prime. Obviously, as there was no Sovereign - there would be no attack at all. The Beacon would just contain some other shit - and Shepard would become a Spectre the way the mission was supposed to go. But Shepard wouldn't be the focus of this story at all. Aside from maybe a few cameo appearances or background news reels he wouldn't appear at all. You would play as a completely different character - one that could be any species. Each species would have a different selection of classes and would have some unique backgrounds to chose from (Which race you chose would also have an effect on civillian attitudes towards you, a Council race would be more trustworthy while a Quarian or Krogan would be treated with suspicion. No matter what character you chose - you would arrive on the Citadel at the beginning of the game with only your Pistol and a small sum of credits. From here on out - it's kinda Skyrim esque. The Citadel would be massively explorable. You would have a main quest - but the approach to the end would be dependant on whichever faction(s) you ally with. A player who joins C-SEC would have a very different path to a SPECTRE or a Cerberus operative. Familiar faces would return - but their lives would be vastly different without Shepard. For example, Garrus would show up as an ally if you join C-SEC, Saren would be an "ally" if you were a Spectre, Tali would be met on her pilgrimage, where you would have the option to help her or ignore her. New faces would also appear - and allies would be optional - you wouldn't have to recruit them all and you would be at no disadvantage for doing so. Limiting the game to the Citadel may seem disappointing to some, But I'd enjoy it more. The environments we've seen in the trilogy were diverse - and we'd barley scratched the surface. You could do so much, from Omega-esque slums to restricted maintenance areas. From lofty Presidium apartments to Industrial zones. Each would be populated by unique characters - who would offer you sidequests (Which would offer the story content of ME1 but without the boring Mako and copy/paste buildings. TL;DR: My ME4 would be very different from the rest of the series.