Skyblock minecraft?

Moin moin, can I help somebody i play a self-made skyblock map which is also good because the bergenzung is not so small. My problem is related to the nether. I would like that the nether is also empty but not a void but that the biomes remain.

Well in the Nether the biomes will remain even if there are no blocks, the void does not mean that there are no more biomes. For example, the Soulsand Valley will still be a good place for ghast farms, the warped forest for Endermann, the Fortress for wither skeletons and so on 😉

Maybe I expressed myself a bit stupid. I would like to make a skyblock map where the nether is empty as well, just like a downloaded world only without the limit of what feels like 300 blocks.

I don't know what kind of limitation you mean exactly but what you could do would be the following. Download worldedit (you have to get them on a server for free with 1-2 slots on the internet, they last for a week) and then you remove all blocks that you don't need in overworld and in the nether. You then drag the map into the single player folder