This has been mentioned elsewhere before, but not here yet:
1) Most important thing to fix, in my opinion, is the bug in the campaign where Vancouver requires a team power of 4300 as this will put a lot of new players off, needlessly. Some people ask about it, lots of others probably just stop playing,
2) On some survivor games, a drone or two is stuck behind a wall after all the other infected drones/mutants have been killed and it becomes a game of "spot the last drone before my time bonuses run out". This is especially a problem for people playing on phones as they have a smaller screen size. How about once the drone numbers drop below a certain point (e.g., 10) the last few drones glow red or blue to make them easier to see?