Corruption/Crimson destroying the Jungle, as well as their fast spreading forcing us to basically create some biome quarantines. Also, RNG in terms of reforging and fishing quests is just plainly awful. We all have seen numerous complaints about players being forced to burn through over 5 platinum coins(assuming a single reforge is around 20-40 gold coins) trying to get that little modifier for your weapon to make it worth using, and getting literally everything but.
I think fishing rewards should be reworked from the ground-up: The player definitely should be able to pick what reward they want based on their fishing milestones(the more fishing quests the player completes, the more rewards become available for them to select). This way, you can get the Fisherman's Pocket Guide you need to finish your Cell Phone instead of getting yet another Sextant/fishing potion/apprentice bait/what-have-you. Who knows, if it works well enough, a similar-yet-modified version of it could be applied to reforges. Much, much better than relying on the RNG(though I do have to admit that having a low amount of money and managing to get Mythical on your Razorblade Typhoon first-try is a wonderful feeling).
As for the Spreading Squad(i.e. Corruption, Crimson, and Hallow), I'd be cool with the idea of them forming basically a single "V" shape at the location the Wall of Flesh was slain, but not spreading past it without player intervention - as long as a corrupted/crimtane/hallowed Jungle is added, of course, otherwise we'd have to pick between one side of the "V" vanishing underneath the Jungle and the biome being potentially lost forever, or it completely obliterating the Jungle when the "V" is created. If it doesn't spread, the player is free to merely get whichever resources they want simply by going to the designated biome area and harvesting what they need, without risking it spiraling out of control, or having to completely purify that biome then re-seed it on a quarantined area.