New maps, improved net code and overall polish are all worthy goals, but for me, I would really like to see more asymmetrical gameplay in the various classes and kits. ME 3's MP did what it could with the limitation that everything had to be based off of a common, human animation skeleton but with the new title's MP I would like to see tangible divergence when it comes to the kits.
For example, I want to see melee kits that are truly melee. A Krogan Warlord brandishing a massive gravity hammer shouldn't be forced to carry a gun; if a player wants to ditch their firearms for a true display of Krogan combat prowess they should be able to. If a kit is designed around being a melee monster, it should have that as the forefront of its gameplay, not just a tweaked melee feature that everyone else has as a throwaway ability.
I would like to see kits that are not based on a humanoid skeleton. Make aliens like the Rachni and the Elcor playable, don't be afraid or unwilling to give these kits unique gameplay mechanics and/or exclusive weapons or amor either. A Rachni Brood Warrior should approach combat very differently than a Human Soldier would, and their methods of attack would be completely different as well. Don't make the Elcor Engineer run around with an Avenger, give them the tank cannons that they wear on their backs; weapons that only they can use.
A perfect example of the type of gameplay I would like to see is present in the game Evolve. In that game the monster plays completely differently than the hunters, and even the different types of monsters play differently from each other. That is the type of asymmetrical gameplay I would like to see. Now granted Evolve is PvP focused when it comes to the monsters and hunters, but applying some of those uniquely diverse playstyles to a co-op setting would be fairly straight forward especially considering that co-op isn't as balance intensive as competitive play is.