I had a thought about a monk subclass that was all about shutting down magic users. Does this sound cool? Here are some rough thoughts on features. 

• An ability that when a magic user is struck with an unarmed strike, the monk can expend 1 to 9 ki points to cause a spell to detonate prematurely. The target must succeed a ConSave or lose a spell slot of a level equal to the number of key points expended. The caster then takes an amount of psychic damage equal to a number of martial arts dice equal to the level of the spell lost. 
  • This ability has no effect if the caster does not have spell slots available at the given ki level. 
• Maybe allow a monk to cast Detect Magic, Dispel Magic, Counterspell using Ki Points. 
• Maybe have a feature similar to the Sorcerer's Flexible Casting ... allowing for Ki to spell slot exchange (both directions)(really only useful for multiclass builds)

Does this sound like an interesting concept? Do you think there is something here?

I've made something along these lines here. The main issue with this concept is it's one-dimensional. The monk will feel useless when encounters don't include spellcasters.

I mean wouldn't they still have all the dope monk stuff? isn't a Shadow monk in the light just a monk? a kensai monk without a weapon, just a monk? 

I mean the shadow monk has abilities that function out of darkness. Kensai are reliant on weapons, but it'll generally be more likely that they'll have a weapon than be facing a spellcaster.

While not a subclass, the Mage Slayer feat would go far towards providing your Monk with good anti-mage capabilities.