Remake the entire trilogy but keep all the soundtrack the same, dont change a thing story wise until ME3
Take Gears 5 and MEA and build the combat system based on those.
ME3 should add the scrapped mission un Kahje, starring Kasumi and Thane, with the possibility of curing the Kepral syndrom, add Miranda to the squad after her visit to the citadel (and change her fucking outfit)
Rewrite Kai Leng entirely, we dont need an annoying and coward nemesis, we need an epic nemesis, Thane should be able to kick his ass (if Kepral syndrom was cured)
Add more lines for Virmire's survivor.
Javik and Leviathans need to feel more organic when added to the story.
Final mission needs to be longer, bigger and with choices and consequences.
I'll add more later on.