I was thinking about a Sun/Moon combo. My idea is to make eclipses. Let's say this Sol and Luna are already part of the game.
-Sol card: pretty offensive, definitely not a support tower, uncommon tower.
-Luna card: support tower, general buff (strengthen wolfs, speed, slow creeps down), unique tower.
It occurs once a tower is between Sol and Luna. For example: Sol(4), Tower(7), Luna(12).
We have it diagonal, it can be vertical and horizontal as well. Sol buffs the Tower and Luna.
It occurs once Luna stands before Sol. Same rule, diagonal, vertical and horizontal works. Sol gets buffed, Luna becomes useless.
Now, the only weak point is, how the system is gonna detect which type of eclipse is occurring. I would solve the problem by asking this: is Sol_position > Luna_position? By that we should have our answer.
As for my opinion, I think this is complicated. I, somehow, like the idea to have the diagonal, vertical and horizontal, buff system. (Hope I explained well, lol).
As the title says this is an utterly incomplete idea.