NEW SUMMONER SPELL: Spell Interruptor: interrupts enemy champ spell

no.

OK, I don't think it's a good idea, but I'll let you the benefit of the doubt.

The important question is: Why?

What effect will it have on the game? What new dynamics/relationship between the support and the rest of the team will it create? What's the benefit of adding that to the game?

Adding stuff just for the sake of change is not a good idea. Change needs to be directed toward a clear goal. Otherwise, it's useless work better spent on more important stuff.

sounds useless

So, basically, a nerf to Fiddlesticks and the few other champions that it'd be useful enough to bother bringing. Wouldn't help the game.

useless against 95% of champ pool, absolutely dumpsters remaining 5%

They don't want a summoner spell to counter a champion because of draft pick; this is why Cleanse does not remove suppression, and this is why your idea would never come to life.