This is a technology-driven field whose advances will parallel those in astrophysical X-ray instrumentation. While we will avoid the temptation for lengthy speculation of what we will find, it is important to say a few words about some of the science issues that will be opened up by further observations.
For the next few years, most of the progress is likely to be made by
the continuing operation of XMM-Newton and Chandra (with
these instruments likely often being assisted by simultaneous RXTE observations to constrain the high energy spectra). The
accumulated body of data from these observatories will allow a
detailed categorization of the X-ray reflection properties of all
classes of accreting black hole in the nearby universe. By applying
detailed considerations similar to those described in
§6.1, such data will allow the first observational
investigation of black hole spin in a sample of both GBHCs and AGN.
Investigation of variability in the iron line profile or strength can
also be studied in more detail than ever possible before. In the case
of AGN, line variability can be studied on time scales down to 
orbital periods (at which point the spectra become too
photon-starved to provide useful constraints) -- line variability
seen on these time scales most likely corresponds to spatial changes
in the X-ray emission from the disk corona. These observatories will
also provide the first constraints on the astrophysical environment of
accreting SMBHs at very high redshift.
After launch in 2005, Astro-E2 will complement XMM-Newton
and Chandra. As well as being able to study broad iron lines at
high signal-to-noise even when observing alone, its ability to provide
very high resolution spectra in the iron line band (thereby completely
characterizing any low-velocity components to the iron line) will be a
powerful complement to any Chandra and XMM-Newton
observation. Even with high-quality data in the iron line band, good
constraints on ionized disks greatly benefit from having simultaneous
hard X-ray (
keV) observations of the Compton reflection
continuum. At present, only the Proportional Counter Array (PCA) on
RXTE can obtain such data. In the near future, our ability to
constrain the continuum shape above 15keV will be greatly enhanced
by the recent successful launch of ESA's Integral (October 2002)
and the future launch of NASA's Swift (September 2003).
At the end of this decade or the beginning of the next, it is hoped that the Constellation-X and XEUS observatories will come on-line. With their very large collecting areas, they will represent a major leap in sensitivity. These observatories will make at least two major advances in AGN research. Firstly, we will be able to produce good X-ray spectra of the highest redshift AGN currently known. This opens the real prospect of being able to constrain the cosmic history of SMBH properties as well as the chemical history of the matter in the vicinity of the SMBH. Secondly, we will be able to study spectral variability on time scales of the order of the light crossing time of the black hole. By searching for detailed changes in the iron line profile, we will be able to search for the ``reverberation'' of X-ray flares, i.e., the X-ray light echo that propagates across the accretion disk due to the finite speed of light [305]. These reverberations signatures encode detailed information about the space-time geometry, and might allow for a quantitative test of General Relativity in the very strong field limit [306,]. The proposed timing capabilities of XEUS could also make a major impact in our understanding of GBHCs. In particular, we will be able to study the temporal power-spectrum of the X-ray variability in exquisite detail. The ultimate goal is to use the form of these power spectra and, in particular, the families of quasi-periodic oscillations that are known to exist [307,294,308], in order to study accretion disk physics and the space-time geometry.
On a longer time scale (maybe two decades), we look forward to ultra-high resolution imaging of black hole systems. The MAXIM concept (§5.3.4) will allow us to image nearby accreting SMBHs (e.g., the SMBHs in our Galactic Center, M87 or nearby Seyfert galaxies) on spatial scales comparable to, or smaller than, that of the event horizon. In some versions of this concept, one will also be able to map out the spectrum of the system (including disk reflection features) across the image. This will clearly lead to major advances -- the geometry and physical state of the disk, corona and jet can be inferred directly from such an image. Combining the spectral information, the dynamics of the relativistic disk can be measured and compared with that expected from material orbiting in a Kerr metric. In particularly clean systems (e.g., Seyfert galaxies) these measurements could be used as a test of the Kerr metric itself -- such a study would be complementary to the Kerr-testing experiments that will be possible using gravitational wave signatures with the Laser Interferometry Space Antenna (LISA).