This paper provides an overview on the rationales in incorporating massive multiple-input multiple-output (MIMO), non-orthogonal multiple access (NOMA), and interleave division multiple access (IDMA) in a unified framework. Our emphasis is on multi-user gain that refers to the advantage of allowing multi-user transmission in massive MIMO. Such a gain can potentially offer tens or even hundreds of times of rate increase. The main difficulty in achieving multi-user gain is the reliance on accurate channel state information (CSI) in the existing schemes. With accurate CSI, both OMA and NOMA can deliver performance not far away from capacity. Without accurate CSI, however, most of the existing schemes do not work well. We outline a solution to this difficulty based on IDMA and iterative data-aided channel estimation (DACE). This scheme can offer very high throughput and is robust against the pilot contamination problem. The receiver cost is low, since only maximum ratio combining (MRC) is involved and there is no matrix inversion or decomposition. Under time division duplex, accurate CSI acquired in the up-link can be used to support low-cost down-link solutions, such as zero forcing. These findings offer useful design considerations for future systems.