The trick is to make the scope limited. Doing an everything-game will be technically incredibly challenging, so start smaller. Choose a location where there are towns not cities, and where not every uber-think exists. Then expand it over time to more scope.
What you need is a setting which has multiple races somehow, but nothing too mega. Some fringe world with bits of several races to start with, and see how it goes. Don;t include titans, make the scope smaller. The God Unit is a land raider or contemptor, or a wraithlord or Battlewagon. As the tech gets better and bigger is possible expand to other worlds via transport or the webway, add new races. I think this is how WoW worked.