Not OP but pretty much everything goes to shit when to meet the hologram kid. If it were up to me I would have like a a couple dozen short ending scenes where some of them get picked and some of them don't depending on your save file content (think fallout ending cards but expanded to entire scenes), and just play hype videos for all of the characters Shepherd is allied with while Shepherd aims the Crucible (which should just be a big orbiting super gun) and blows up Reapers with it.

That ending is pretty straightforward but I think that's all you need to pay off the series and give people a satisfying conclusion - you can have alternate versions of scenes like where characters live or die and then choose what happens based on your total war prep score, then you could have like a big set of conclusion scenes based on who was alive and who was dead and what their relationship to Shepherd was etc. Think of the structure like a visual novel scene web.

You don't need anything super complicated, as long as you gesture at the decisions made in Mass effect 1 and Mass effect 2 like by having a couple different end cards that could play depending on the fate of the Racni Queen and stuff like that, then I think it could have been a benchmark ending for game series with multiple entries.