Been thinking about what ME4 might be like and I think I've got a badass idea for it. Hear me out.
It's a reboot set many years after ME3, like hundreds of years later, where the galaxy is in a peaceful state. You play as a commander, tasked with exploring uncharted space, reaching out to new alien races or finding new planets as a means of the council expanding in the hopes of advancing further. Being in uncharted space gives the excuse of introducing new characters, new species, new weapons, new planets and even a new threat. A technologically advanced alien race that is dominating that galaxy. The story is of what it's like to have these characters and a crew of various alien races you already know of (Asari, Turian, Salarian etc) and have them all be in this fish out of water scenario where everything is new, the politics and history is all new and have them be caught up in the events as this galaxy is under war. You're commander of the best ship that they can find, a giant cruiser ship on a scale that is the same size as the Citadel. That can act as a hub world in between missions where you can wander around and communicate with your crew, go on side missions etc.
A prequel may tell a good story if done well, but to me it doesn't really generate much interest considering I know what state ME3 leaves the series in so it will always be in the back of my mind when I play it. I think a reboot is kinda necessary.