I would like to keep Counterspell (the new version) but I would be quite happy if Mage Slayer forced a con check to cast a spell on its reaction effect.No.
You're proving my point, if anything. We should be going back to there being ways to interrupt casting as a NON CASTER, not tripling down on a terrible boring spell that only lets Sorcerers, Warlocks, Wizards and the nerdiest of Bards interrupt spells (oh and two of rarest Paladin Oaths, look out).
Reject Counterspell. Return to interrupting spells the real way, by hitting casters in the face.