Forthcoming large galaxy cluster surveys, from missions such as Spectrum-RG/eROSITA, will find hot, X-ray luminous clusters out to high redshifts. Short snapshot observations with a new X-ray observatory with capabilities similar to those planned for the Constellation-X mission should then be able to identify a sample of ~500 hot (kT>5keV), suitably relaxed systems; and later re-observed them with longer exposures (~20ks per cluster on average) to measure the X-ray gas mass fraction, fgas, to a precision of ~5 per cent. We examine the ability to constrain dark energy using such an fgas sample, and find that the fgas experiment offers a competitive and complementary approach to the best other large, planned dark energy experiments with a comparable Dark Energy Task Force figure of merit. If time permits, I will also present preliminary, new constraints on gravity at large scales using growth of structure measurements. These constraints are, importantly, complementary to the distance measurements from the fgas technique discussed above. For this analysis we use a convenient parameterization of departures from the current gravity theory, General Relativity, and present-day measurements of the growth of cosmic structure in X-ray luminous galaxy clusters from the MACS, BCS and REFLEX X-ray cluster samples.