Feat: Mage Slayer

There is a nifty feat titled Mage Slayer which seems clearly intended to make melee characters effective at shutting down casters or at least noticeably better at that task.

By RAW the Mage Slayer when he/she is nose to nose (within 5 feet) of a caster can poke that caster as a reaction to a spell cast.  Disappointingly, the mage slayer must politely wait for the cast to complete before making his/her attack.  

That wait can be rather disappointing if the caster decided to polymorph the mage slayer into a slug, teleported away, or any of a myriad casts that disable the mage slayer's ability.  

I'd think the mage slayer wouldn't patiently wait for the spell to complete, he or she would smack the caster and hope to fizzle that spell before it completes.  This posting considers how to make that work in 5E (hint: a house rule).

Before trying to improve the situation, lets make sure the RAW is in front of us, so here it is:

Feat: Mage Slayer

You have practiced techniques useful in melee combat against spellcasters, gaining the following benefits:

• When a creature within 5 feet of you casts a spell, you can use your reaction to make a melee weapon attack against that creature.
• When you damage a creature that is concentrating on a spell, that creature has disadvantage on the saving throw it makes to maintain its concentration.
• You have advantage on saving throws against spells cast by creatures within 5 feet of you.

Lets suppose we just add a single additional bullet to the RAW ability:

When you damage a creature in the act of casting a spell, you may force a (normal) concentration check to complete the spell. 

The use of the word may is important in the above, as the mage slayer may force a normal concentration check on the spell being cast, which will cause it to fizzle and drain a spell slot -or- wait to completion and force a disadvantaged concentration check to break a concentration spell.  

I'd go a bit further and allow the Mage Slayer to stack a free "identify cast" in the same manner used for counterspell as this is the melee equivalent of that spell. This is outlined in Counterspell and Knowing What is Cast posting.