[mwcard=MW1J16]Mordok's Obelisk[/mwcard] already does this, sort of. And it's best played by a solo (or single buddy) Mage who isn't going to be affected much him/herself - like the Forcemaster.
Sure, Naiya still channels net +1 while in an aquatic Terrain, and it doesn't affect Conjurations and other spawn points and familiars that channel (like Sectarus and Libro and the Siren's one), but any creatures that pop out of them get taxed, so it penalizes the opposing player in much the same way. There are a few objects that don't get taxed by the Obelisk, like Battle Forge and Construction Yard. But I could imagine an affordable Enchantment that is basically a reverse [mwcard=MW1E20]Harmonize[/mwcard]; would probably need a similar mana cost to Harmonize, too.
