The built-in or included diffusers aren't really meant to provide nicer light. They're just there to help cover a greater angle than the widest zoom setting. An optional flash diffuser is generally larger and designed to have a more versatile effect.
Metz makes a couple of flash diffusers designed for their flash units. The Bounce Diffuser 54-23 is a soft, velcro-mounted bounce diffuser that is described as matching all three of the flash units mentioned here and would probably fit on anything. There's also a plastic clip-on diffuser, Mecabounce 58-90 for the 48 AF-1 and 58 AF-1 models and Mecabounce 44-90 for the 54 MZ-4i.
Of course, there are many third-party diffusers like the famous Sto-Fen Omni-Bounce which will fit any of these flashes. However, the 48 AF-1 manual mentions that it automatically detects when the Mecabounce 58-90 is attached and displays a wider focal length automatically — that probably doesn't work with third-party diffusers.
Sigma also makes a bounce reflector designed for their flashes, but which would probably fit on anything of a similar size.
External Power Packs
The Pentax AF540FGZ's recharge time can be improved with the TR Power Pack III, which uses six C-cell batteries.
And the Metz 58 AF-1/AF-2 and Metz 54 MZ-4i can both use Metz's NiMH-based Power Pack P76. They require different power cables, though.