As I said before on this topic, swapping out fx on skills is essentially a skill-by-skill basis. We have to incorporate support for fx-swapping in the engine each time we introduce it to a new skill type. To do it for every skill would be…a lot of work to say the least. Then there’s the fx work on top of that.
The secret boss’s spell effect is actually a really hacky spell I made years ago that turned out decent rather by accident. Mechanically Doombolt’s fx and that one are very different.