Keep in mind that you can pile up tons of loot on your existing world then save your character and take that character to a new world if you had to generate a new one.

Then you immediately build some chests near spawn to store all your stuff as you will be overloaded. Build shack, chests, move stuff around.

Now you have your "new" world, but you can keep the same seed so things are familiar and you will have lots of advanced stuff so you can breeze through the earlier parts and get caught up pretty quickly.

Mods can help in this regard by adding extra storage slots for your dude if needed and I think some can even capture your buildings and help restore them on a new world.

You would only need to start a new world if you explored the biome's that the new content spawns in. If you perchance have unexplored area's that have the correct biome's, the new content may spawn there.

Hope that answers your question.

Nobody knows if we'll need a new map for Mistlands, but for Hearth and Home and for the new mountain update it's as Tannah says. As long as you haven't been in that area of the map before, all the new stuff will spawn there.