Short answer: I don't foresee an issue with bringing back Smooth.
Longer answer: The main thing that made Hexany work is that "discarding" cards sounds bad, but often is not. This is for two reasons:
1. Discarding unseen cards from your draw deck really isn't much of a penalty at all. It feels like you are "losing cards," but it's not that bad. Discarding from your *hand* or *in play* hurts you because you lose resources you could use. But a card on top of your deck isn't one you could use -- and there is no mathematical difference between "X was placed in my discard pile from the top of my deck" and "X was shuffled to the bottom of my deck at the start of the game," until you exhaust your draw deck. All that happens is you draw other stuff sooner.
Note: downloads do affect this analysis a bit; if you are forced to discard something you might have downloaded you did genuinely lose a resource you might have accessed. Same with deck manipulation.
2. There are a lot of ways to access your discard pile; even OTF-legal things like Nanoprobe Resuscitation and Fal-tor-Pan. Palor Toff is the ultimate example; if you have a huge discard pile, Palor is basically a "wild card" that can become anything you want at Interrupt speed. You have the resources of a giant hand without being vulnerable to Scorched Hand; and it's way easier to dump cards into your discard pile than to draw them, because draws are widely recognized as powerful; discards are actually thrown onto cards as a "penalty" or nerf!
As a result CCG designers habitually overestimate how bad a "cost" it is to discard cards off the top of your deck (Data, Keep Dealing is a Decipher-era example; Habit of Disappearing a more modern one). This isn't a STCCG-specific thing either; in MTG, witness Bazaar of Baghdad or Reanimator decks, for instance. In a lot of games the discard pile is a *resource.*
With this in mind, there were two linchpin cards that made Hexany work. Pre-errata Beyond the Subatomic can easily fill your discard pile (as a "cost" for getting a card you want), and Palor Toff let you use that discard pile as if it were part of your hand. As Allen mentioned, proportions were critical in Hexany decks. Olav's original article
had twice as may BtS and Palors as the others; Franklin's 2002 Worlds
deck had even more extreme ratios, 30 BtS and Palor and just a few copies of the others. As long as you have a BtS and Palor in your opening hand, you can throw enough cards into your discard pile that Palor can *become* whatever other Hexany card you want
Which brings us to the current day: BtS has been (wisely) errata-ed to remove cards from play, making its "cost" a genuine cost and not actually a powerful benefit. And Palor is OTF-banned. Which means the key parts of Hexany are gone -- the remaining cards either have a once-per-turn limitation and/or are OTF-banned themselves (All Threes is IMO still too powerful, it's an Interrupt-speed Kivas). So there's no easy way to guarantee access to whichever Hexany card you want at any given point. Without pre-errata BtS and Palor, Hexany as a deck concept is dead. You can still sprinkle in the individual cards, but there's no worry about sitting back and watching the opponent fiddle around with their deck ad nauseam.
Kris raises a good point about whether we *want* to bring back this kind of mechanism. Even though I agree it takes away time from the main game, I'm not super concerned -- it only takes a few seconds to resolve Smooth, it's once per turn, and without BtS/Palor you don't have constantly looping Hexany plays.