There is a proof of concept somewhere about floating islands, with a custom terrain generator that part is definitely possible. It wouldn’t be a simple mod and as far as I understand it wouldn’t be compatible with other biome-altering mods (or at least not other ones which alter the terrain generator… I’m not quite sure of how it works, but @BrunoSupremo would know and is good at explaining this kind of thing.) But it would definitely be possible to have a map based around floating islands.
Unfortunately the current water tech doesn’t allow for infinite waterfalls flowing off the edge of them, though, since there’s no way to replace the water which flows off. It’s something that’s planned for the future, but we don’t have any timeline for expanded water features just yet. Once we have source blocks and drains/sinks which can destroy water, we’ll be able to do all sorts of awesome construction involving things like waterfalls and aqueducts.
New trees, animals and ores are all fairly easy to create, so you may want to look at the StarterMod and use it as an example/template. Anything that you can “copy and paste” from the vanilla (unmodded) game, with a couple of tweaks to make them unique, is a great place to start out with. There are plenty of modders who are happy to help out with advice and assistance, so I’d fully encourage you to try it yourself
As for the temples, you can either add new building templates or you could possibly add some new enemy camps as a way to spawn the temples in. Or, you could even make the temples into large terrain features (like how the bunny statues work, or Bruno’s rock formations in the Canyon Biome mod), although I believe they use a smaller scale than the terrain and building construction so you’d probably have to use a trick to make them match up – it shouldn’t be hard, you just need to make sure that the models are scaled up to be the same size as a building made from “building voxels” rather than “item voxels.”