Here's a list consisting of 18 of the most promising/popular indie MMOs in development. Most if not all of them do not have a release date, but some of them (Crowfall, Gloria Victis, Fox Hole) are getting closer to launch. This list has been posted elsewhere on the web. If you have suggestions to improve it, please let me know.
Ashes of Creation [Alpha]
Setting: Medieval Fantasy || View: Third-Person || Focus: PvP/PvE || Budget: ~$40M* || Engine: Unreal 4 || Origin: U.S.
Nodes system: player participation influences the type of content in the surrounding areas
Open world housing: players have the ability to survey and develop land anywhere in the world
Castle sieges with hundreds of players with many moving parts
Harsh, unforgiving and stunning environment
https://www.youtube.com/watch?v=V_hsJPB8y7k&ab_channel=AshesofCreation
Corepunk [Alpha]
Setting: Cyberpunk || View: Isometric || Focus: PvE/PvP || Unity 5
Seamless open world with fog of war
Players explore the world, participate in trading, complete dungeons, or engage in PvP battles
Day / night cycles as well as weather conditions
Flexible character customization with a wide variety of builds
https://www.youtube.com/watch?v=1_RPcB-9rE8&ab_channel=IGN
Mortal Online 2 [Beta]
Setting: Medieval Low-Fantasy || View: First-Person || Focus: PvP || Engine: Unreal 4 || Origin: Sweden
Open and realistic sandbox world where player houses can be built almost anywhere
True, real-time combat where outcomes are decided by skill rather than levels or gear
Legendary crafters trading across the globe, famous warriors studying the art of combat, explorers puting names on the map
Individual reputation: people will recognize players by their impact on the world
https://www.youtube.com/watch?v=hTWLlefiy2E&ab_channel=BadmanLasermouse