3.9 is the end as far as it goes to 68k AmigaOS.
Now, "legitimate" ?? By that rule you would have to "downgrade" to 3.1 since thats the last one by C=/Amiga and 3.5&3.9 were little more than a collection of 3rd-party hacks.
The only current development would be AROS-68k but I'm not sure wether thats allready at a level where it could be considered an upgrade.
