For the swamp, I would make a bunch of structres that are the little patches of land, and only spawn in your swamp biome. You would also want to set the generated top layer blocks to water. As for the floating islands, make the islands be structres that spawn in your biome, in the air. Set the biome base height to the lowest (-2 i think) and the varyation to 0, with the top layers being air. This gives a sorta floating island effect. To make a plateau, set the height varyation to the highest, or use structres.
On MCreator there should be a section to add new biomes. Is that what you are after or do you know that and want more freedom in it?
For the swamp, I would make a bunch of structres that are the little patches of land, and only spawn in your swamp biome. You would also want to set the generated top layer blocks to water. As for the floating islands, make the islands be structres that spawn in your biome, in the air. Set the biome base height to the lowest (-2 i think) and the varyation to 0, with the top layers being air. This gives a sorta floating island effect. To make a plateau, set the height varyation to the highest, or use structres.