Heavy Storm would be too much. There are already so many Pendulums that benefit from being destroyed by your own Wavering Eyes, and Heavy Storm achieves that and more. Sure, it doesn't add another copy to your hand, but you get rid of any defences your opponent may have had. We have Twin Twisters and I think that's enough. Harpie's Feather Duster is, ironically, not as good as Heavy Storm can be. But while Raigeki and 2x Dark Hole can exist in a place where many monsters are hard to destroy, Spells and Traps (bar Pendulums) are a lot more devastating when removed.
I'd actually love to see Redox back at 1, because that would be cool. But for sure, if any of the Dragon Rulers are coming back it would be Blaster or Tempest.
And then lastly, I'd like to see either Monster Reborn, Premature Burial, Snatch Steal or Brain Control make a return. I know, they can all steal games if you draw them (or Hidden Armory), but isn't that what Yugioh's about? Beat your opponent's before they beat you, or prepare to face the heart of the cards. And to be honest, with so many decks able to OTK, the moments you get to use the Spells will be a lot less powerful than they used to be. The biggest downside to Reborn and Prem is the ability to make even bigger combos, and in fact secure an OTK you may not have had without it. But, if that just makes more decks powerful, I'd actually be fine with it. I like playing hand traps anyway.