This game has come leaps and bounds in terms of variety needed to beat the game. From the new obstacles, to the smarter AI behavior, to the new types of ships both we and the AI gets; we have had to learn more unique ways of dealing with these challenges. However, these new obstacles are pretty much only relevant in the early and late games.
In the early game, you have very little to your name and such need to spend wisely to survive the initial onslaught and still be able to take over more planets. In the late game, both you and the AI have gotten lots of "cool toys" to work with, and now you must find a way to out maneuver the AI with what you have. That is, can you kill their home with your cool toys before they kill your home with their cool toys.
However, in the mid-game, the AI does not have a lot of threatening variety, and you can out-power most AI planets. As such, building a "blob of death" and capturing planets in the exact same way dominates the mid game. Sure, you will need to pull out clever tricks for those pesky Mk. IV planets (which, means this is the most fun part of the mid game), and you need to be able to handle special obstacles, (like black hole generators and raid engines), these are quite rare.
Thus, I think something should be done to improve the necessity of variety in the mid-game. Again, it is so much better than it used to be, but it still has a way to go.
Note, I have no idea what can be done about this. I am not an expert on applied game theory or maintaining pacing in RTSs or anything like that.
EDIT: I forgot to mention. The lack of variety of ship options availability to you in the mid-game also contributes to this problem.