(from other thread)
I think if you want to add a limit on use of cantrips as follows, it would be fine, and wouldn't change their use overly much:
"You can cast cantrips a number of times equal to your [spellcasting ability score], until you are too mentally drained to cast a cantrip again without a short rest."
So most of the time, you can cast 16-18 cantrips until you need to rest an hour to be able to cast it anymore.
You can even add an optional grim and gritty version:
"You can cast cantrips a number of times equal to your [spellcasting modifier], until you are too mentally drained to cast a cantrip again without a short rest. You can cast more than that without a rest, but each cantrip you cast beyond your [spellcasting modifier] without a short rest, you must make a Constitution saving throw equal to your spellcasting DC for that spell, or else take 1 point of temporary Constitution damage."
That, or you can add a misfire chance to it after a certain point, or a fizzle chance after a certain point. I believe there is a spell misfire chart already in the DM's advice booklet.
Edit: Yup, here it is, just apply this to any cantrip cast after your [spellcasting modifier] without a rest, if you fail a Con saving throw:
MISHAP d6 Result
1 A surge of uncontrolled magical energy deals 1d6 psychic damage per level of the spell to the caster.
2 The spell affects the caster or an ally instead of the intended target, or affects a random target nearby if the caster was the intended target.
3 The spell takes effect at a random location within the spell’s range.
4 The spell’s effect is contrary to its normal effect, but is neither harmful nor beneficial. For instance, a fireball might produce an area of harmless cold.
5 The caster suffers a minor but bizarre effect related to the spell. Such effects last only as long as the original spell’s duration, or 2d10 minutes for spells that take effect instantaneously. For example, a fireball might cause smoke to pour from the caster’s ears for 2d10 minutes.
6 The spell activates after 1d12 hours. If the caster was the intended target, the spell takes effect normally. If the caster was not the intended target, the spell goes off in the general direction of the intended target, up to the spell’s maximum range, if the target has moved away.