[Fantomen]

The transitions between early, mid and late game i usually refer to as a nations "power curve". I think one of the most important things when building a strategy is to outline what kind of early, mid and lategame strategies you want to use and when and how the transitions will take place. You need to prepare for mid game during early game and for late game during mid game, while at the same time fielding competitive forces against your enemies. These preparations can involve getting the right boosters to build a path for casting that global at turn X, or remembering to research that specific summon that will work with your battlemages when you get evocation Y.

You need a plan for your research, your site searching, your expansion, your battle tactics and your gold/resource needs. The trick is to find a way to make all of them synergize with your nation as well as each other. The more you can make them synergize the sharper your power curve will be.

Fast expansion is generally a good thing but not everything. There are many nations for which a second fort and more research mages is worth more than a couple extra provinces.

I would advice you to start with following guides that describes the nations power curve in a linear and pedagogic way that is easy to follow. For example Baalz MA atlantis guide, Jim Morrisons Fomoria guide or my own Agartha guide. I'm not saying those are "better" than other guides, just that I myself found it easier to start with that kind of guide. Those three would also get you into three different concepts: Teleporting thugs(atlantis), Sacred troops(Fomoria), and big armies with buffs and summons(Agartha)

Then when you have a general Idea of what the stages of the games can look like move on to more complex things like blood nations, communions, dominion pushing, fatigue strategies and so on.