Hmmm, crud, you're right on the combo system as it currently stands. That will have to be taken out, you're right. I had wanted to do a longer-term combo sort of system to keep players from just spamming a single spell AND to make it so that twitch reactions aren't needed for that sort of thing, but the differences in spell power really destroy that whole approach irrevocably. Damn. It was borderline too complex of a mechanic anyhow, and adding on more rules to try to fix it would just push it squarely over into the territory of too complex.
On the bright side, I had already been planning on a shorter-term twitch sort of combo system. That would still have the same sort of low-power/high-power risk, but that could be worked around, I think. Mainly because if these modifiers were a special effect of certain spells (such as dousing with water in your Magicka example), then that could work well. Of course, I seem to recall hearing that a lot of those combos in Magicka, while fun at first, became game-breakingly unbalanced pretty fast in the hands of players. That's the problem with open-ended combo systems.
I still want to do something along those vague lines, but it will require more thought. Both of the two models I'd been thinking of, including the one I'd implemented so far, were too rife for abuse. Perhaps the notion of "combos" will mainly be limited to spell crests, we'll see. That allows a lot of spell customization and splitting and so on. Will have to think on this more -- and thoughts on potential models are of course welcome as always. Coming up with one that is simple, fun, and not exploitable will be a challenge, but I think it's worth putting some brainpower into.