I thank all the answers and advice. I have no intention of creating a mod, although I understand the technical differences between a mod and a complete set. In the same way that 0 A.D could have been a mod and preferred to be a game with its own identity.
Thanks for your answer! I understand what it means that the game is in alpha state and hence my interest in whether the engine is sufficiently separated from the game as you can update it as you being improved.
Besides, I have no intention of using a proprietary engine such as Unity.
The truth is that my game differs 0A.D enough to avoid making a mod. In fact, my intention was to make my own engine. But I found good idea to try, first, to use an engine already.
As I answered above, I have no intention of making a mod. On the one hand, because part of the goal is to learn about game engines. On the other hand, because my game is different enough to make it worthwhile to have its own identity. I know it takes time. It is not the first game that I make, nor the last that I will do. But as Yoda says, "Do or do not. There is no try".