In the server space, SAS, U.2 and U.3 connectors are mechanically compatible with each other and partially compatible with SATA connectors. U.3 is probably the

In the server space, SAS, U.2 and U.3 connectors are mechanically compatible with each other and partially compatible with SATA connectors. U.3 is probably the dead end for that family, but they won't disappear completely for a long time. Traditional PCIe add-in cards (PCIe CEM connector) are still around and also not going to be disappearing anytime soon, but are in decline as many use cases have switched over to other connectors and form factors, particularly for the sake of better hot-swap support. M.2 (primarily SSDs) is in rapid decline in the server space. It may hang on for a while for boot drives, but for everything else you want hot-swap and better power delivery (12V rather than 3.3V). The up and coming connector is SFF-TA-1002, used in the EDSFF form factors and a few other applications like the latest OCP NIC form factor. Its smaller configurations are only a bit larger than M.2, and the wider versions are quite a bit denser than regular PCIe add-in card slots. EDSFF provides a variety of form factors suitable for 1U and 2U servers, replacing 2.5" drives. The SFF-TA-1002 connector can also be used as direct replacement for the PCIe CEM connector, but I'm not sure if that's actually going to happen anytime soon. I haven't seen any sign that EDSFF or SFF-TA-1002 will be showing up in consumer systems. Existing solutions like M.2 and PCIe CEM are good enough for now and the foreseeable future. The older connectors sometimes need to have tolerances tightened up to support higher signal rates, but so far a backwards-incompatible redesign hasn't been necessary to support newer generations of PCIe (though the practical distances usable without retimers have been decreasing).