Is it one of those "Everyone has the same race" Castle Builders?
I was going to make one back in the day where everyone chose a "God" whom they had to keep the favour of (With sacrifices, tributes, killing x enemies, whatever.) and in turn they're granted a few "God Powers" a la Age of Mythology and an overall bonus (Ranged unit damage increase, or something.)
And then if you fall out of favour with the God, bad things happen.
Or something.
Random events are always fun. Both good ones and bad ones (And something other than an earthquake.)
Plagues, space debris, aliens, dragons, tornadoes, mass exoduses, revolutions, discover a new resource-thing. You could do almost anything, really.
Oh. And an idea that just jumped into my head. In most of the Castle Builder-ish maps I've played, populations (and thus income) are based on the amount of houses you build. But if you make the population a separate number, it'd be cool if every x seconds, the population takes away AKA "eats" however many of your food (Lumber?) and then you get income based on however many people you had enough food to feed (Because the other ones are dead.
That way you'd have to hunt animals or fish or farm to keep your people alive so you can get income. Slightly more realistic, I think.