I would not have this count opponent's spells or any abilities at all. I like the idea of this, but on removal spells it doesn't feel right, since it's hard to 'prime' with another negative spell you can make use of, without wasting the spell or killing the creature. I'd prefer this on positive effects, where stacking positives is a lot easier.
So I block your Hill Giant with my Oreskos Swiftclaw, you Giant Growth it, and with the Giant Growth on the stack I Primed Bolt it and two for one you? That seems like very poor game play.I think, at a minimum, it needs to have been targeted by a spell or ability you control this turn (just like Heroic). Repeatable abilities, though, probably totally break the mechanic, so probably it needs to just trigger on having been targeted by a spell you control.That makes this pretty narrow, which leads me to the same place I go with a lot of my comments, which is that I think this is something that would be better as a handful of cards than something done in volume. This is definitely an interesting space to play with, and I think there are a lot of ways to toy with it, e.g. Final-Sting Faerie variants, etc.
I agree we want to chop out the cards that punish targeting, and so focus on rewarding players for targeting their own stuff multiple times. I'm optimistic it can be balanced around activated abilities (particularly things like Akroan Mastiff). Scaling effects like Prime Hydra will break with old infinitely-repeatalble effects like Outrider en-Kor, but I suspect that a strong combo like that won't ruin Modern as long as it's not an auto-win.