I've kept the game interesting for myself by creating and then playing my own mod.
When I play, I kind of feel like I'm playing as all the teams at once. When the AI plays well, its because I taught it how. When they make a mistake, I try to correct their thinking for next time. At the end of each playing session I check the AI logs to see whether their big decisions made sense. At the end of each game I think about what I could have done differently, and what each of the other players could have done differently, and even how the game itself could have been different - to be more fair, more interesting, more fun. And then I try to make it happen!
So yeah. Mods. and in my case, making the mod as well as playing it.
(My mod doesn't dramatically alter the gameplay like, say RoM. My mod is basically just a polished version of Beyond the Sword.)