Long Time no updates, which does not mean I abandonned the project.
In the meantime, I was able to find a solution to do the Level Conversion ...
This is the new FPGA-2-IEC-Board with the Level-Converter (5 Signals) on top:
(BTW: Those two boards are the first PCBs I etched myself using the method described here: http://thomaspfeifer.net/platinen_aetzen.htm)
Further I have found a new FPGA-Board with enough I/O for nice Stuff like a Cartridge Port or real Joystick ports. As the new Board does not have a VGA-Connector, so I need to make a new VideoConverter-Board too. This should keep me busy for the rest of this year....