"Diego Stutzer" in m...

Now the hard part. How does one create an Impedance, which decreases
"slower", for frequencies close to zero but then decreases "faster" for
higher frequencies, than the simple parallel RC-Circuit?
Is there some kind of procedure like the one for syntesizeing LC-Filters
(Butterworth, Chebychev,..)?

What you are asking about is a form of what's traditionally called the
network synthesis problem (creating a network of components to realize a
prescribed signal response) and specifically the synthesis of a one-port, or
impedance.

At one time (when phone companies ruled the earth and computers had
conquered few signals and DSP was reserved for BIG things like the US
Perimeter Acquisition Radar at Concrete, North Dakota -- affectionately the
"PAR"), this was a popular subject in engineering schools at the
advanced-undergrad or graduate level. It is still extremely important
sometimes, especially with the sophisticated signal processing used today on
continuous-time signals in consumer products. A host of
applied-mathematical techniques (Foster and Cauer synthesis, Brune's
impedance-synthesis lemma, etc.) apply even to one-ports. Some of them are
highly counterintuitive. Not, in other words, a subject perfectly matched
to the contraints of brief advice on newsgroups. (Note also that
Butterworth and Chebyshev approximants are mathematical methods to approach
one group of curves out of things that naturally give you a different type
of characteristic -- "Butterworth and Chebyshev" have nothing to do with
specific circuit topologies or components). If you want to pursue it
further I could suggest investigating "network synthesis." Temes and
LaPatra had a reasonable modern (1970s) book about it. Karl Willy Wagner
started it all in 1915 by inventing filters.

Richard Clark suggested also investigating the small op-amp "biquad"
networks for designable frequency response (actually you can turn them into
one-ports, the so-called shunt-filter class, but again a bit of a subject
for a brief response). Note that technically a "bi-quad" is any network
giving a biquadratic transfer function (2nd-order numerator and denominator)
though in RC-active filters it's often applied to the closely related
Åkerberg-Mossberg and Tow-Thomas configurations. For practical info see van
Valkenberg's excellent general introductory book on filters from the 1980s.

For an accessible modern example of these small op-amp-based "biquad"
networks, look up the LTC1562 from Linear Technology, a commercial chip with
four trimmed "biquad" networks, programmable by outboard components for
applications from a few kHz to a few hundred kHz.