As a roblox developer, it is current impossible to give out clothing for completing badges. This is a power that is only given to games temporarily and during events and developers have no control over what the items the badges reward are.
My proposed improvement is to add a system where a dev can spend robux in order to upgrade badges to reward badges, that give out a t-shirt, shirt or pants, or other ugc item (robux price of upgrading the badge based on reward type), where we can give custom made items to our players for achieving badges. This would let us have meaningful rewards for players who complete difficult quests, invite lots of friends, or play for long periods of time. Developers can also just give out easy badges that give out clothing with the games logo on it, so that players can get a nice shirt for free and developers get (semi)free advertisement from players wearing the shirt.