The old Master of Magic did have things like conjunctions that affected the magic of the land. So the idea is valid, however I think in this case things would change too quickly. Every turn/season would have different bonus. There would be too much chance of twisting it. "Let's see, I control 3 fire shards already, so let me also wait two more turns until summer to attack to maximize the bonus.

But idea of seasonal variation on the look and feel of the world would be an awesome addition.

Of course, having said that, I feel that seasons are almost too long of a time period, and more so as we get later in game. A sliding scale like the Civilization series does it is nice. The first 50 turns are equal to one year, then the next 100 are equal to a season, then move to months and weeks even. But thing will also need a minimum time to complete, like children will always take 25 years to grow up, so once you get to seasons instead of years on the time scale, growing a child will take 4 times as long.

you might be onto something there OP... perhaps i would want to wait 1 more turn for my attack if i'm mostly a fire shard guy and want to have an extra effect during the summer!

This is what I was thinking when suggesting it. And also why I said a nominal bonus, not a huge one.  Just enough to make you think about holding things off for 1-3 turns, but not so much that you ALWAYS will hold off because the bonus is so awesome.

I also like the idea of changing the time scale (more turns per season), but then you'd be waiting more turns for your children to mature.