Self healing speed from Medkit should not be nerfed.
But making all items that survivors bring into the match spawn more chests. The time it takes to find chest and unlocking would be far greater than 8sec debuff.
At the moment, if you dont bring a Medkit, there is really no reason to find chest, unlock, get the medkit to self heal for 24sec; that cost at least half a Gen time. My idea would nerf Medkits that brought into the match further, while keeping Medkits in chest still useful.

Comments
I like this idea. It also works as a shadow buff to Hoarder
Or remove all current bloodweb items from chests and replace them all with a stronger "chest-tier" items which you are not allowed to keep like Flashbangs. I think this would make the game more interesting and fun while buffing chests a bit.
Perks could also be changed to help with that, as Risidual Manifest, Scavenger and Pharmacy would also give you the strongest version of an item in the game and hopefully increase the pick rate.
Some items and addons are op. But items in general were not a problem, its the no-time required to get items through searching and unlocking.
If you find a Commo toolbox in a chest, its fair; but not if you have it at hand at the beginning. Thing is, chests in the trial rarely give you powerful item, in the opposite items you bring in is fully customizable