If you want to stay OEM, BMW have just released their"black" series of soft luggage. Tank/tail/soft panniers.
If you want aftermarket, the Kreiga (spell) bags are simply excellent. I was part of a touring club and most of us had these tail bags. 5, 10, 15 and 20 litre options from memory. Most have had them on their bikes forever. Loads of them have done over 250,000 klms. My mate Cliff is on his third ZX10. All his have done over 200,000klms each. All with the same 15 litre kreiga tail bag. Waterproof, sit well and hard wearing. 👍👍