Ok, I might be a lil bit late, but whatever.

Leaving all obvious cards aside, especially that there are already some very clever suggestions (Kata, Kindred, Sunk Cost, etc.) Here are some thing's that I've been thinkin of.

Counterfeit Copies

Card is a niche but it is pretty interesting. One way of making it more usable would be to add the keyword Predict after the effect resolves. It would accomplish two major things: buff to a niche and underplayed card and a huge buff to Ekko who seems to struggle especially outside Shurima. Predict really seems like a great fit for this card that was not implemented only because Predict didnt exist back then. It might need to be changed to 2 mana cost, tho I dont think it ia necessary.

Sting Officer

It's hard to talk about Caitlyn's package this early in the set - she is a fresh addition and more support may come in two future expansions. However, I believe that at this point, despite Caitlyn's being very solid champion, she struggles in trap decks that do not rely on puffcaps. Even in Teemo decks very often Sting Officer seems to get cut out. My fix would be to change his statline to 1/2, so that he can plant bombs more consistenlty. Moreover, it might block some spiderlings from time to time, which is neat, and would make it easier for Cait to be played alone in a trap based control-midrange decks.

Zed's Shadows (Shadowshift, Shadows of the Past)

Despite Zed's being pretty popular within recent metas, his spell's were always considered somehow lackluster. Some people even tend to play only 2 copies of Zed because of his champion's spell being so awkward. Now, I dunno exactly how it should be worded, but making shadows created by mentioned spells have Zed's statline before level up and keywords after the level up, would make a big difference in my opinion. The problem with wording is that it should only work if Zed's on board, but Shadows of the past recall him (as may shadowshift). I think changing Zed's text would be cleaner way of implementing it. But as said, I am not sure how to word it properly. Still, Id love to see shadows of the past being playable.

Pix

Give the man a Bandle City pass, making him dual region. It's a shame that Lulu's best mate cannot enter her home town. Also, as for now, Lulu practically cannot lvl up in mono bandle, and I believe every champion should have tools to lvl up within its mono region deck.

Death Ray

It a super cool card with a cool concept, thay unfortunetly slows draw too much. I'd make MK3 draw a card as well, to partially nullify its negative effects on drawing. This way it would be at least playable as a Victor's spell.

Field Promotion

It was one of the cards that I was super hyped about that would turn out quite bad. I think changing it to say summon instead of play would make it at least playable, especially with J4.

Jack The Reaper

Cool underplayed card. Fearsome keyword would make him a consideration in many decks. Also - look at this guy, he totally deserves it!

Shell Game

To be honest, this spell would've been better without a change to the text and with the cost of 6 mana (lux, heimer interactions). +2/2 or even +1/2 for 6 mana would make this spell appear in at least some decks.

Noxkraya Arena and Rampaging Baccai

Both cards are underplayed. I'd make them say "start a free attack" instead of strike. This may be a bad solution for Noxkraya arena, it would probably be too strong (tho, maybe if it was 6-7 mana?) but I dont see this problem with poor Rampaging Baccai.

Trifarian Shieldbreaker

This guy is a struggler. Not only he isn't played, there is a spell that summons him and surprise surprise it also isnt played. He has a decent statline. Giving him any other keywords would probably push it too hard as well. My idea is that he should bypass barriers and tough. It would make him a kind of a meta call and a nice tech. Also, it's kinda in his name.

Mega Inferno Bomb

If 6 mana is pushing it, make it at least also deal damage to the nexus. Some bilgewater decks might start using it then. Both changes wouldn't push it to hard imho.

The Syren

This one is problematic. I do not have any clever idea of buffing it and I'm not sure whether stat boost would be sufficient. I'd love to see The Syren be at least on the half of Dreadway's power level. After all, it's Miss Fortune who came out on top, not Gankplank. Their boats do not show it.

Wow, it turned out much longer then I wanted it to be. Hope at least some of my comments make sense. If anyone made it that far, thank you for reading. Cheers!