Don't get me wrong, the levels are great. Alot of fun and very detailed. However, I think the game could really benefit from some larger, more open levels in the future. Nothing huge, but take the first New York chapter for instance. When you exit the mall you have to find the subway. You walk out and just around the corner, there it is.
Imagine if you had to look around a block or two before finding it? It would open up the level immensely, giving more options for exploring.
I don't know if it's even possible, without tanking the frame-rate, but it would be cool regardless.
A mutator option would be a-ok with me. The regular game is really fun, but it does feel a bit anti-climatic to only defeat "standard" waves of zombies. I'm all for having the option of including bosses though. The more gameplay options the better.
The weapon upgrade system makes no sense. It is way too easy to get the tier 5 upgrade. So unlocking anything between tier 1 and 5 is a huge waste, and any sense of weapon progression is lost. It should be an accomplishment to unlock 10 extra rounds for your magazine, or having your power and accuracy increased. Right now you just play a few maps with the same gun and you'll earn enough credits and unlock tier 5.
Once in a while the Rocket Barrage will simply fail to deploy. It will be called in and be confirmed, you'll hear the rockets being fired in the distance but they never arrive. I have no idea what causes it but it is super annoying to have your most powerful ordinance bug out, often.
Restricted areas really need a revision. Like you say, on some maps, areas close to the objective is out-of-bounds for some reason while others aren't.
Personally I'd like to see them, if not entirely removed, then at least massively down-scaled. Maps feel way smaller than they really are because so many routes are blocked off to restricted areas.
I get your point, I really do. I don't agree on it though. Doors in the game, much like real life, are bottlenecks. They are extremely dangerous to traverse through, and I think the game signals that well. Insurgency thrives on these realism elements to make the virtual battlespace feel a tiny bit more real. A map without doors, doesn't convey that.
Also, since when has map balance been a priority? Sinjar anyone?