Challenges are good only for people who are grinding levels and are absolutely worthless for people who are done with grind.
Why not make challenges give Challenge Reward Points (CRP in short)? Let's say that finishing all challenges for given week reward you with 1 CRP. You can use CRPs to get rewards like: gun skins (something like cc skins, where you can't apply camo), uniform camos, custom models for guns (Like different stock, barrel etc., you can apply camo, something like M4 Prestige skin or support pack SCAR-H skin).
For example:
Uniform camo: 1 CRPs
Gun skin: 3 CRPs
Custom gun model: 5CRPs
That way even people who are done with grind can get rewarded for participating in challenges.
[EDIT: Challenge Points changed to Challenge Reward Points due to @bitter cairn crying about it
]
Alright now it's fine