Translation:I want the duvet to cover the whole bed.
Could you in swedish just say "Jag vill täcket att täcka hela sängen. instead of "Jag vill att täcket ska täcka hela sängen." The english is I want the quilt to cover the whole bed.
What about "Jag vill att täcket täcka hela sängen"? Is it incorrect or unnatural?
No, that doesn’t work either. You need the ska. Whereas in English you say: ”I want the quilt to cover”, you have to say ”I want that the quilt will/shall cover” in Swedish.
It's very formal and it's even recommended not to use it even in formal texts these days, but it should still be an accepted answer – I've added it here now, if it's missing elsewhere, just report it via the Report a problem button.