Transitional duvet
The transitional duvet, or all-season duvet, serves most people for most of the year, depending on the filling material and body type, and is only supplemented in winter. It is one of two duvets in the 4-season duvet (transitional and summer duvet).
Cotton and merino wool do the job excellently and are sufficient for most people.
If you need a lot of warmth, go for camel down or even cashmere*.
In winter, you can add a summer duvet (or a second all-season duvet) that you simply put on top.
*(Individual item, cannot be returned free of charge.)