Ideally I want to be able to run not only text-oriented PDP-11 software, but also system and gaming software from Soviet BK-0011M home computer that had 512x256 monochrome video mode and 256x256 4-color video mode (with some predefined palettes). Output might be VGA with 2 resolutions - 640x350 (to display mentioned above 512x256 and 256x256 with some border space around) and 640x400 (to display "extended" video modes 640x200 monochrome and 320x200 4-color when every line will be shown twice by hardware) - both resolutions have the same pixelclock 25.175 MHz and the same field frequency 70.086 Hz. These are some photoshopped examples:

It was existing BK-0010 and BK-0011M software and now - new modes:

640x200 monochrome (first) and 320x200 4-color (second)

It's even possible to create very tricky videomode for 320x200 where every line may have its own 4-color palette...