I really hope that Larian succeed with this project. It would be sad for this to go the way of N-Space and Sword Coast Legends. Us D&D fans can be harsh and unforgiving. I am hoping that Larian and Wizards can work well together to deliver on this. Transparency and managing fan expectation is key. Negativity aside I am actually really hyped for this.
For me, I want it to have some of the same features that gave NWN1 and NWN2 the longevity they had and still have. Players are still making content for both, despite support ending years ago. What made those two games last as long as they did was the toolset and ability to direct connect to servers via IP address. Meaning that even after the Gamespy matchmaking servers went down people could still do multiplayer. So for me the three main things are as follows:
Private hosting(direct connect to IP)
For me, other than 6 character party this would be my biggest request as well. A tool set to make big arse long customized content would be amazing and potentially timeless.