The paper provides an overview of ultra-dense cell-free massive multiple-input multiple-output (CF-MMIMO) networks and addresses key challenges and open questions for their future deployment in 6G. CF-MMIMO is highlighted as a promising technology to meet the ubiquitous connectivity and growing data traffic demands in 6G. The authors first survey state-of-the-art research on CF-MMIMO and ultra-dense networks, then discuss key challenges such as low-complexity/scalable resource allocation, fronthaul limitations, massive access, synchronization, and channel acquisition. They also address open questions, considering different design comparisons and suitable methods to tackle these challenges. The paper aims to provide a roadmap for future research directions in this area, facilitating the development of CF-MMIMO for 6G. The discussion covers practical infrastructure, signal processing, capacity bounding techniques, massive access, resource allocation, synchronization, and calibration. The focus is on sub-6G bands due to their superior propagation characteristics compared to mmWave or higher bands, which are expected to handle traffic in 6G. The paper concludes with a detailed mathematical model for user-centric CF-MMIMO and a discussion on the practical implementation of CF-MMIMO, including the challenges of network densification and the potential benefits of high network density in 6G.The paper provides an overview of ultra-dense cell-free massive multiple-input multiple-output (CF-MMIMO) networks and addresses key challenges and open questions for their future deployment in 6G. CF-MMIMO is highlighted as a promising technology to meet the ubiquitous connectivity and growing data traffic demands in 6G. The authors first survey state-of-the-art research on CF-MMIMO and ultra-dense networks, then discuss key challenges such as low-complexity/scalable resource allocation, fronthaul limitations, massive access, synchronization, and channel acquisition. They also address open questions, considering different design comparisons and suitable methods to tackle these challenges. The paper aims to provide a roadmap for future research directions in this area, facilitating the development of CF-MMIMO for 6G. The discussion covers practical infrastructure, signal processing, capacity bounding techniques, massive access, resource allocation, synchronization, and calibration. The focus is on sub-6G bands due to their superior propagation characteristics compared to mmWave or higher bands, which are expected to handle traffic in 6G. The paper concludes with a detailed mathematical model for user-centric CF-MMIMO and a discussion on the practical implementation of CF-MMIMO, including the challenges of network densification and the potential benefits of high network density in 6G.