WoW's Shaman has one of the simplest interactions which is rather satisfying to pull off:

Flame Shock dots can proc Lava Burst.

Lava Burst is a very slow cast, high resource spell that deals a lot of damage. It's slow cast and MP cost make it inefficient to use by defaul. But Flame Shock's tick damage ( A spell rather similar to WHM's Dia, with upfront damage and a DoT) has a chance to make Lava Burst instant. Not only that, Lava Burst will always cause a critical strike on a target suffering from Flame Shock's dot.

It's such a simple interaction and yet it gives enough variability to give breaks to Shaman's allways be casting healing gameplay.