The discovery of gravitational waves (GWs) of frequencies around 100 Hz, and the recent strong evidence of a gravitational wave background at nHz have been two of the most important discoveries of recent years in Physics. Expanding these two frequency bands will bring new invaluable information about the cosmos and gravitation, and has triggered the community towards new concepts. Among them, new generation of CMB detectors to search for the lowest possible frequencies, LISA to cover the mHz band, or the use of cutting-edge sensing technologies to search for frequencies above kHz. Yet, these ideas are not able to cover all possible interesting physics cases, and the search for new ideas is very active.