I think you should keep this as a mod. In my opinion there is no reason to start creating a new engine for this or anything like that. Continue developing with A2: CO and when A3 gets released it shouldn't be too hard to make the move. Using the A2/A3 engine gives this the right feeling all around. Also, let the community influence the development but keep your own ideas. In addition, it should be allowed for the community to host the servers themselves if they want. You could start taking donations and if that doesn't go well, a reasonable one-time fee would fit this well. Keeping this as a mod ensures that it stays unique and "hardcore". Many great ideas have been ruined by trying to make them suitable for more people, there is no need for that on this one.