I spose if your using an engine, your limited to make the game how it works, but I still think you should draw up a plan how you would actually go about twisting the engines capabilities to make what you want, technically. I even mean the simplifications your going to be making too, like perhaps no facial animation and no fingers, even that should really be decided.
I am planning on using Realm Crafter Pro, it is affordable, and with Pro I can customize a lot of the scripting.
Most of my notes center around what I plan to change from standard MMO's and how, or what features I plan on to try using from other games. Like making persistent characters similar to The Sims, and what problems I might run into and how to fix them.
I see many games get a feature so well done, but the rest of the game is horrible. Like how Pirates of the Burning Sea, their ship combat was awesome, the rest was so so. Or how A Tale in the Desert IV has great crafting, or Wurm Online has very cool farming and animal husbandry, even their resources are done superbly. Why not put many of these features together, true, effort and money are good reasons.