I agree with you in one point : the character's customization is low, you have to choose 5 perks, with 5 (or 4?) choices by perk, that's all.
PS: the Extinction mode is good for the time the dev have spent in it. (I'm sorry for my english D:).
Ideas for the mode
- possibility to earn/find perks (fast hands, etc.).
- improve the teamplay side : you can do the game alone with turret+explosive bullets+armor.
- add a survival mode in a sort of area, a bunker or something else like that where the players have to stand and fight for their life.
- add more weapon accessories, dropped by bosses or hidden in a secret place which isn't always open.