OFDMA is the easier part and doesn’t bring much in case there are few concurrently active clients of same WiFi AP … and the big difference will be mostly slightly better RTT with lower delay jitter … the difference between OFDM and OFDMA is that the later can service multiple clients at the exactly same time, during same transmission period. Radio-wise both are the same (same modulation), it’s signal processing which is more complex for OFDMA. Not something unheard of though, OFDMA is part of LTE since very beginning almost 15 years ago.
The MU-MIMO is a harder nut. The essence of MU is beam-forming and the more professional systems offering it (later releases of 4G and 5G) feature antenna arrays, e.g. 8x8 antenna array. Without antenna arrays beamforming is not effective and thus MU-MIMO can not work. Such antenna arrays are not small either, a well designed 8x8 array for 5GHz (and up) will be something like 1m x 1m panel with individual X-pol elements spaced 2 lambda apart. And add additional space for weatherproof case. Not-so-decent (and effective) indoor designs might come with 0.5m x 0.5m wall or ceiling mount antenna array. Anything smaller will have mediocre results at best.
My guess is that full-scale MU-MIMO deployment will be in point-to-multipoint deployments where the hub location will feature (large) antenna array while clients will keep using traditional (MIMO 2x2) directional antennae.