Key features for my preferences:
1 - D&D 5e rules with all core character Race/Class/Sub-Class/Background options, and whatever Feats that can be implemented properly.
2 - Updates to include new standard options as new books are released.
3 - All the iconic monsters must be available.
4 - Party management. I need the option to control a party of characters in solo mode, or to adventure with others in multi-player mode.
5 - Multiple viewing modes to choose from. I like to switch between 1st person, top-down and over the shoulder views depending on the situation.
6 - DM mode is a big plus, especially with the tools to customize and host private servers, similar to the Neverwinter Nights games.
7 - A large sandbox map for solo play, with expansions adding new areas to explore. I would prefer the Greyhawk or Forgotten Realms settings, but Eberron and DragonLance would be good too.
8 - A large number of well-written quests of a quality similar to The Witcher 3 would be a great bonus to the sandbox mode.
9 - Horses and other mounts are a must have. Playing a Knight or a Paladin without a trusty steed is depressing.
10 - Animal Companions and Familiars would be great.
And I want all of this for FREE !!! ;-)
Well, I suppose that is asking too much. I would gladly pay top-dollar for a game that included these features.