What do you want your next body to do that the F3 doesn't? That's the question that needs to be answered.
If you like the F3 and want another like it, get another F3.
The FA and F4 will matrix meter your manual Nikkors. That's an advantage.
The TTL on the F4 is far more advanced than the F3's first-generation flash technology.
The FM family are less expensive / just-as-durable backup bodies for the F3, if that's all you're wanting.
Some prefer the FE2 to the FM family. That's always an option. Stay away from the original FE - their SS viewfinder indicators are connected to the shutter speed dial with a single thread that is prone to break - mine has multiple times).
The F2 (or even an F) will give you similar pro options to the F3 without the need for batteries - and have a completely mechanical shutter.
The list goes on and on.