Yeah, I mean, realistically it wouldn't change much, since you still need to beat Plantera to open it. How about this idea, then? While fishing in the Dungeon, *after Plantera is beaten,* you have a very small chance of reeling in a Biome Lock Box of a random type. The Biome Lock Box can be open with a Biome Key of the corresponding type, and you'll get the corresponding weapon. Additionally, however, a Biome Lock Box can be shimmered to turn it into a Biome Key, meaning that theoretically you can get both keys and weapons by fishing in the post-Plantera Dungeon.

The probability of reeling in a Biome Lock Box is exactly twice as rare as reeling in an Extremely Rare catch. In numbers, this means that your chance of reeling in a Biome Lock Box is equal to fishing power / 9000, and it's hard capped at 1/12. Practically speaking, this means that if your fishing power is 100, your chance of getting a Biome Lock Box is 1.11% (1/90), and if your fishing power is 300, your chance is 3.33% (1/30). Of course, you have to consider that there are 6 different Biome keys, so while your chance of getting *any* Biome Lock Box is generally going to be between 1 and 3%, the chance of getting the specific one you want is much lower. And you also have to contend with the post-Plantera Dungeon enemies, such as Ragged Casters, Giant Cursed Skulls, and Diabolists.

This would be by far the greatest odds of getting a Biome key, but the significantly reduced control over which one you get and the added issues of fishing being often slow and constant harassment by difficult enemies doesn't make it easy at all. I think this would be balanced and fair, as well as giving people something they want.