I often find when I play long-haul campaigns on 4x games that either I get overwhelmed in the early-to-mid-game, or it goes so well that by the late game I'm too overpowered relative to my rivals for there to be any challenge or fun in it, and based on discussions with others that's quite a common problem. The most obvious solution is to allow the player to be able to change the difficulty level after the campaign is underway, so if that's not something the devs are already considering I'd like to suggest it. Obviously some people will use it to cheat, but to be honest it's their prerogative anyway to play however they want to in order to get the most enjoyment out of the game.