Source

• counterspells in each nonblue color: green.
• added rarity

Preferable would probably be a counterspell based on large creatures rather than many creatures to pay tribute between the difference between green and white.

Currently a colorshift of Unified Will.

Your alternative idea sounds similar to Spell Rupture and Stubborn Denial.

This is a pretty hard sale to make.

Maybe something like

> Heartbeat

> Instant

> Counter target artifact or black spell.

?

I'm not fond of color hosing in general and not make such blanket use of abilities not in a color's core to get there. Green shouldn't get to destroy a Grave Titan just because it is black, so it shouldn't counter it either.

The reason I avoid Spell Rupture is that in the original thread I have put rhystic counterspells secondarily into white as "taxing" effect (extension of Mana Tithe).

I suppose I could go for "Counter target spell if its converted mana cost is equal to or less than the greatest power among creatures you control."