Let me play as a Turian mercenary and do cool vehicle missions in other galaxies, and let there be an advanced character creator and a male quarian team member. Let the ship be smaller and less clean-looking than the Normandy.
If they absolutely have to scrap a race, I'd prefer to have them scrap the yahg and/or vorcha, if they're not going to use them anyway, or the Asari, but that's only because I never liked the asari.
Have us go to the home planet of a species with a bunch of alien flora and an unbreathable atmosphere where we need to wear special gear to go out, that would be so cool.
If we run into enemies in space, I'd like real space battles this time. I understand that you couldn't fight the reaper vessels in mass effect 3, but I'm going to assume that there are going to be no reapers in the new game. Maybe we could even have small missions that you really need to control the ship for, like in star trek online. I love the space battles in STO, much better than ground battles.
Please let the infiltrator really sneak, and not have the enemy be alerted as soon as the team enters a room, so you can sneak attack for extra damage. This way I have a good reason to make my team out of a bunch of infiltrators sometimes. In Mass effect Infiltrators were just sharp shooters that turned invisible sometimes, you rarely got to sneak attack because one of your party members would always alert the enemy or the enemy would miraculously just know you arrived. It would also be nice if more options in a quest were class specific, this would make people more curious about trying other classes.
It would also be nice if we did not need a full party to go on missions, that could be optional, much like in DA2, so you can try to solo most of the game and then brag about it to every one. The easiest class for that could be infiltrator, if you are capable of hiding and covering your tracks, of course.
If in a party, I think the PC should have a bleed out timer, like in ME multiplayer, so it makes more sense that you can bring team members back to the fight and so it doesn't look like everyone just gives up after the PC is down.
Oh, and I would not like a prequel that much, nor would I really like cameos from mass effect.
that's pretty much all I could come up with.