Well if you like your 600 and you are not planning on keeping it then the Fz6 seems the logical replacement. If your looking for an upgrade in power then the FZ8 or FZ1 are good options but they are both the exact same size so only difference is the engine.
Depending on your riding preferences, style and budget there are plenty of options out there. The MT07 is a great city bike and is stupidly cheap even brand new, the MT09 is a great replacement for what you have although a very different beast.
Obviously other brands are available haha but basically it just comes down to what your wanting to spend 👍