Pretty basic tweak on Rupture Spire/Transguild Promenade. This can be played as one of those, or to filter ->C on the turn you play it.

It's pretty much strictly better than those, because it also has a couple other uses: tapping for mana then letting it die (if you really need to cast that spell this turn), or tapping it for mana then bouncing or flickering it. It's obviously still fine power-level-wise, but it might be more confusing than the printed version.

Whoops. Edited because I was wrong.