Quite the combination. I'll jump right into it.

Many players look at Silver Keys and have the brief thought of "wow, I wish I could afford that" before moving on. Additionally, some players open Lockboxes only to find another Spike Mohawk, and would really rather that they get their crowns' worth from the boxes.
My solution to both is a variety of Key types. At their most basic, they would consist of three; Bronze, Silver, and Brass/Gold.
Bronze would be the least expensive of the three, available for 300CE in the Supply Depot. Consequentially, it would also apply a modifier to the RNG of the box it opened; items considered "rare" would show up less than normal.
Silver keys would not change.
Brass/Gold would be available either as a Token reward, a Prize Box item, a Casino wheelspin, or a random drop from Lockboxes. Alternatively, it would be purchaseable for 1kCE on the Supply Depot. This Key would apply a modifier to Lockboxes to make them drop a slightly smaller range of more expensive items.

-Cannonball Shield-
A Gunner-oriented Shield that consists of a miniature cannon with a wooden, stone, or iron shield on the side. It provides decent Normal defense, and grants a charge time reduction of low->medium as it alchemizes to higher Stars. Its most prominent feature, however, is that upon executing a shield bash, the wearer moves backward about twice the distance of the Magnus line's recoil.
Further, while propelling the user backward, the cannon on the shield will fire, releasing a medium-velocity cannonball around the size of a Callahan charge. The cannonball will deal Stun 100% of the time, but will not pierce enemies, instead knocking them out of the way similar to an actual Shield Bash.

I like the latter one better, but the former was made in response to someone complaining about key prices.