I recently got into Extinction, coming as a long time zombie player ever since W@W, and found how overly frustrating it can be with the number of weapons you can acquire and then have to cycle through. At times I have 5-6 different things and depending on how many phantoms or rhinos are chasing me, it can get quite crazy.
I think they should add in a weapon wheel like RDR/GTA V. Just hold down Triangle/Y and the wheel pops up and boom you can instantly select the weapon you need. Have certain items such as riot shield, so flam, and Venom X keep dedicated spots so that it won't be confusing with a random wheel of weapons.
Another idea would be quick buy ability. Instead of pressing on the d-pad and than having to confirm the purchase which takes valuable time, hold down the d-pad button of the selected item you wish to purchase and it automatically buys it. Could help with armor/ammo in a pinch and won't interrupt the double tapping features associated with each class upgrade.
Let me know your thoughts. Hopefully these can be implemented in a future update.