If you're looking for a homebrew option... how about this? You can scribe a wizard cantrip into your spellbook, at half the cost of a 1st-level spell. But, in order to use the cantrip, you have to prepare it, and it counts against your limit of prepared spells. (While you have it prepared, you can cast it at will, as usual for cantrips.)
Wizards are always pushing on the limits of their prepared spells, so there's a real cost to this. At the same time, you aren't giving up any arcane firepower - you still have as many slots as before, you just have fewer options in how you use them.
More versatility in your cantrips, against less versatility in your leveled spells, is much easier to balance than trying to weigh versatility against power.