The RPG elements of ME1. The emphasis of more personal stories and bonding with your crew of ME2. The shooting combat of ME3 and Andromeda. Set up armor and customization like God of War. Each piece of armor has mod slots. Most slots are just basic stat boosts but the special mods actually give you some new gameplay mechanic. Being a biotic and certain mods gave you special biotic powers would be a neat change.
Also, as much as I despise Andromeda, the free-form classes really works. Also, movement and gameplay were fantastic (character combat did move a bit too fast, but whatever). Exploration was good, but it needs to be more honed in and focused like in the first ME.
Haven't even thought about the story. Not sure how I feel about some new big bad.
If we even see this game, I'm thinking around 2028.