Shmendan
Steampunker
I think a platinum coin portal would be very helpful. Of course not in pre hardmode, or even early hardmode, that would be too overpowered and would allow you to buy basically anything. I think the platinum coin portal should be activated once you have beaten all three mechanical bosses or have beaten Plantera (what do you think? Post in comments.) Even after you have met the requirements, it should still be very rare and would only drop 1-2 platinum coins. I think it would help out when you are trying to save up for some late game things, such as the clentaminator or the autohammer. When you are mining for chlorophyte after fighting the mecha bosses you will likely come across many pots, thus increasing your chance to get it. If someone could make a graphic and post it into the comments that would be neat.