NO stupid RNG Store, period! Let you improve the gun of your choosing, and if they want to charge a cool mil per upgrade then I'm good with that, but none of this random bs among a pool of 100 weapons.
I think the basis of ME3 MP needs to be there, but I would like if they could make the game more team oriented with kits that perform certain roles, roles that are needed. It was great that you could do solo, but overall the games success was the coop portion. How to do those roles I'm not sure at the moment.
Better spawning mechanics, I don't want to see a spawn just appear and then some asshat nukes them with a missile. Aka, no missile nuking speed runs. It takes no skill and trivializes the threat and skill needed to actually defeat that in normal means. So when Cerberus spawns come in, I could see a shuttle make a hot drop, with door gunners shooting the ****** out of everything with cover fire, Atlas's that should be aerial dropped to a spot, even better if AI can decipher what the best spots are to add difficulty.
Add in some aerial units to be shot down, and small vehicles.
No stupid RNG store.
Only 1 space rat to limit annoyance.