There's been some chatter about restricting some subset of Mental Misstep, Gitaxian Probe, and Gush. I wonder if it'd be possible to print some hate cards that slightly reduce their viability instead.
One option is a Trap mechanic - an instant card with a powerful effect but high casting cost, where the trap triggers when your opponent plays a card without paying mana. But this is really hard to get right - it has to be good enough to discourage Gitaxian Probe, but not so good that it makes Force of Will unviable.
A hate card that doesn't destroy a single Force of Will, but rather grows in strength the more free cards your opponent plays, would be preferable. I'm thinking something like this:
T: Add U to your mana pool.
T: Until end of turn, you may cast target instant or sorcery card in an opponent's graveyard. You may not use mana to pay the card's casting cost. If you cast the card, exile it.
I think this would make for quite interesting in-game decision-making and interactions. When your opponent has Force of Will in his graveyard, any decision to tap Hate Island, whether for mana or whether to cast your opponent's Gitaxian Probe, becomes a weighty one. A skilled opponent can play through the hate with good planning, timing, and bluffing. Snapcaster and delve make for interesting situations.
An alternative is to make it target instants and sorceries with converted mana cost 1 - that limits the hate to Probe/Misstep and not Gush/FoW/Mindbreak Trap, but also then hits the card draw package of Ancestral, Ponder, Brainstorm, Preordain. I think I prefer the level of skill and interaction that would arise from being able to target Force of Will, but targeting card draw instead might have its merits.