I think the idea of making the homeworlds themselves harder to destroy is flawed. Yes it is "interesting enough" to crack the defenses of the planets during the game, but what makes the game interesting overall isn't the victory of destroying a planet, but the decision about whether or not it will be "worth it" in the grand scheme of things. The same sentiment keeps being repeated: I am having the most fun at the times when the AI is threatening to win, when the current AI attack is difficult and complicated to deal with. Also, why should the AI focus so hard on defending its first home world? It's smart, it should understand that losing one home world isn't the end of the game, but that it represents the threat of the game ending. For this reason, I think attacking an AI homeworld should not be a final test of the player's ability to destroy a planet's defense, but it should represent the AI's final, last ditch effort to eliminate the player all together. The unleashing of the special forces is one step in this direction, but in my opinions, it does not go far enough. Imagine this:
- Most of the strategic reserves do not unload on the home world itself, but in various locations around the galaxy, similar to how a CPA forms.
- The bulk of the reserves are held until the first home world is destroyed.
- The special forces are not unleashed until the first home world is destroyed.
- When the first home world is destroyed, several warpins begin. Not just where there is a warp gate, but in several random player controlled planets. We know the AI can do this, it just doesn't commit the resources to it -unless it deems the player a legitimate threat-
- The second AI home world becomes temporarily invincible.
Now you have something that is hopefully interesting to deal with. Suddenly, the AI home worlds don't -need- to be nearly impossible to crack. If you can deal with this attack, then you may easily take the second home world. The AI tried to beat you and failed, so there is no longer any need to drag it out with a turtle fest.
It could even be taken a step further. What if the constant warpins never end? Either you survive until the invincibility of the second homeworld expires and you somehow have enough of a fleet to spare to make a final strike before the AI finishes you off, or you lose. Increasing AI progress by a big number sort of accomplishes this already. Just make that number bigger, and add a timer that stops the player from killing both home worlds at once.