One of the key tasks of the Global Geodetic Observing System (GGOS) is the provision of high quality and consistent global terrestrial reference frames (TRFs) to accurately monitor and reliably interpret important geophysical processes such as the global sea level rise. The GGOS requirements of 1 mm accuracy and 0.1 mm/yr stability in the long-term are not achieved so far by the currently available TRFs due to, e.g., the lacking of an optimal architecture of the ground networks of the Satellite Laser Ranging (SLR) and Very Long Baseline Interferometry (VLBI) space-geodetic techniques. In this study we address this issue by assessing the merit of enhanced SLR and VLBI networks. We simulate all four space geodetic techniques (Doppler Orbitography and Radiopositioning Integrated by Satellite - DORIS and Global Navigation Satellite Systems - GNSS besides SLR and VLBI) contributing to the TRF in the time span 2008-2014 and combine them by applying simulated local ties (LTs). From the combination of SLR and VLBI we conclude that the precision of the LTs should be 1 mm or better. Simulating LTs with systematic errors indicates that the stations Wettzell, Badary and AGGO play a key role in the combination and should definitely be precise to 1 mm or better. Simulating enhanced VLBI networks, which are expected to be operational in 5 and 10 years from now, demonstrate that the anticipated network in 10 years fulfills the 1 mm GGOS goal in terms of the TRF defining parameters. In the end, our simulation tool allows to assess the impact of the LT accuracies and their monitoring intervals and to identify beneficial new co-location sites to improve the TRF in order to achieve the GGOS goals.