Instead of reactionary spells, why not just add some spells or permanents that make things cost more or trigger counter measures automatically? There was a thread about "trap cards" before, my suggestion there was to require a price (Like 3 random quanta) to to place any card face down (To add on to the surprise factor, minus the mark and pillars you're using) and then having to pay another price (whenever possible) for the effect to activate automatically when your opponent meets the trap card's requirements?
sounds like a good idea to me. That way your opponent would know that you played a trap(and if they were paying attention, perhaps even the cost) but not which specific trap you played, at least until after the first time the trap is triggered. Effects could include simply damaging or destroying the first creature to trigger it, causing some creatures to not attack or even to attack their owner instead, or even destroying/damaging only creatures in certain slots. A trap which for example destroyed all attacking creatures in even numbered positions could force entire new strategies on what order to play cards in(as well as subtly nerfing Fractal since that would cause a player to play several of the same creature all at once, half of which would then die).
