Alyssa, my most advanced character, hasn't yet fought Plantera. I've nearly finished her base, organized all the storage, and so forth, but not quite. The Crimson and Hallow on her hardmode world are both perfectly contained, and there are fishing huts in every useful biome. I can't very well abandon all that.
But I don't want to take a Terra Blade, Flame Wings, Chlorophyte and Turtle Armor, and so forth into a brand new world either.
Best compromise, I figure, is to keep what I have but also make a look-alike for one of my mobile characters and put her into a new world. A brand new character but a somewhat experienced player, if you see what I mean. Ought to progress pretty fast, as far as I've made it so far in other worlds at least.
I'll try to resist the temptation to fly to the wiki before puzzling things out a little… but probably won't be able to for long.