This is what I would like:
1. I would first and foremost like a continuation of the things that make Mass Effect so great to begin with (i.e great characters, excellent storytelling, fun combat, choices & consequences, moments of both victory and defeat, awesome universe with multiple species and deep lore).
2. Exploration & discovery. Multiple large areas to explore that represent various worlds - some that involve the main quest, but some that are strictly exploration and side quest material (like in ME1, but much better).
3. Meaningful negotiation and diplomacy - trading favors, competition, making friends, making enemies, discovering new species & etc.
4. A sense of world building. If you find a place to colonize, I would like to see the results of colonization. We are after all trying to get a foothold in a new galaxy (I assume).
5. An epicly awesome & large main ship - Ark or whatever you want to call it. I'm talking like something that feels like a city in space (think Novigrad from TW3 and Citadel from ME1 mixed together but more enclosed because it is a space ship).
6. A secondary ship that is really the pathfinder's ship. The Ark shouldn't be the PCs home, just an awesome place to resupply, relax, and etc.
7. At least 15-20 main missions. Please a more dense main plot than was offered up in DA:I. Branching stories would be great, but aren't required. Again something like ME1 where some main missions can be done in varying order would be awesome.
8. More variety in combat for the various classes, and variety within classes with more abilities than a player can use at one time.
9. More customization of spaces, items, outfits, and etc.
10. A sense that more is going on than just what the PC is doing. I would like the main character to be a part of something, not the center of something. No more saviors of the galaxy/continent. Just a guy or gal who is one part of a larger venture, and ends up involved in a great story.
11. A relatable and worthy antagonist.
12. Next gen visuals. Great soundtrack. The Vigil Theme from ME1. That song is required if it is to be Mass Effect.
13. Something similar to DA:Keep that reflects some of the choices in the prior games (Save the Council or not, whether some side characters were helped or not, Genophage, Quarians/Geth and so on). The final choice doesn't have to be included.
14. A wartable like feature that allows us to send groups of squad mates off on other missions - but not timed missions, let it be they're gone till the next time you complete a mission/assignment and return to the ship/base/whatever.