cut card allowance in half like academy. Only 3 lvl1's allowed, and only 2 copies of everything else. Might shake up your local meta with completely gutting many books or strategies.
This would have no effect on almost any of my spell books - except swarms, which would have to swap some level 1 creatures for different level 1 creatures.
If your goal is to hurt the guy playing 4x Elemental Wands and 6x Nullify, remember that every time you remove one with Crumble/Dissolve, you come out ahead in both mana and spell book points (since he loses the attack spell, too). There's also Curse Equipment and Explode.
In fact, with some of the redundancy (e.g. equipment that does basically the same thing, alternatives to Block, alternatives to Nullify, etc), I'm not sure limiting to 2/3 or even a "Highlander" does much other than push swarms out - unless you also restricted certain expansion sets (e.g. all Academy products, which then bans Wychwood Ironvine, the pants slot, Arcane Ward, Disperse, Crumble, Curse Equipment, Iguana Regrowth, Gator Toughness, Redirect and similar, etc).