In the campaign when you collect Nano catalyst you can use them to purchase Suit Upgrades.
But After you have purchased all the upgrades Nano Catalysts usefullness drops to 0. They just pile up since they have no more usefulness.
I have a concept for the Nano catalyst.
A Crynet Supply Store.
These will be on numerous places on different levels Each with Unique Content and will be only unlocked after finishing the campaign on any difficulty.
What you can get from these things.
Weapons: Weapons that you can find on the ground or used by enemies will be available and a/some weapons that are rare or only can be purchased through stores.
Ammunition: Ammunition/Explosives.
Nano Suit Upgrades: Same as what you get now although different stores will have different upgrades.
Nano suit Power cells: These are a one use only abilities granting you timed abilities. Example, Maximum NanoSuit will give you Double the Sprint Speed Higher Super Jumps and Super Melle. Vehicle Drop Abilitie to mark a drop zone for a tactical Jeep or APC.
Marine Support: Call in Some Marines for Additional Support!
Purchase Vehicle Knnowledge: Purchase Knowledge to be able to drive certain Vehicles. These must be purchased before being able to purchase specific Vehicle Drops.