So here is a bigger list of changes I thought about targetting not only the meta but also the underplayed cards to make them better, have a read.
Twisted Fate:
Change his lvl up requirement to "I've seen you draw more than 1 card in 4 different rounds".
Add a perk where the first time TF slays an enemy unit each round, you draw 1 (removed on lvl 2).
The main problem with him is that he's too easy to level up for the value he provides afterward, but without a lvl up he's basically useless and easy to remove, changing his lvl up requirement so that it's no longer possible to lvl him up in a single round makes it so that the enemy actually has time to play around him. Adding a perk that forces TF into combat also makes him extra vulnerable to removal, but extra rewarding when done right. Now players get to choose whether they stick to having a lot of safe card draw, or use TF in combat to remove enemy units and draw cards on top. With this change, TF's quick attack will actually become useful, and since you essentially need to draw 1 extra card each turn after the round you play him for the lvl up requirement to count, it opens up synergies with regions with any sort of card draw, OR combat potential, so P&Z is not the only option for lvl ups anymore. The exact numbers are up to debate and need playtesting, but I think the idea of changing his lvl up requirement to "I've seen you X in different rounds" is the way to go in general.
Aphelios:
Not much more to say here, moon weapons themselves are pretty much balanced by being slow speed and the unit itself does not engage in combat before leveling up anyway. Increasing the cost prevents players from curving from him on turn 3, to TF on turn 4 as well, which is important. If there was anything else, I'd say make it so that moon weapons themselves no longer count towards phasing a new weapon, but if that was to go through then I'm not sure if the cost should be increased, either way, it needs playtesting.
Viktor:
To put it simply, the hex core upgrade card will no longer have a set cost, instead, its cost will depend on the keyword chosen by the player so that applying elusive is more expensive than something like fury, for example, it will be a similar mechanic to invoking, where the randomness is still a thing, but the player has much more control, and thus can pick different ways to play depending on the situation and strategy. For the lvl up, I think he should have a more difficult requirement but a better reward, numbers are up to debate, but remember since Viktor will have to see you play created cards, removing him resets the progress.
Nocturne:
For as good as Nocturne is in theory, his lvl up requirement basically forces you to play with Targon to have access to enough nightfall units, and replacing that with Fearsome will actually let Nocturne be useful in more than like 1 or 2 decks. I think we all can agree that after leveling up, Nocturne works as a win condition, but it's just so unnecessarily hard to do so. Changing his requirement to SOMETHING that revolves around Fearsome units instead of nightfall will make him much better and still fit the theme.
Fiora:
This will simply make it so that once she levels up, she will have to kill 4 more units instead of 2, bringing up the total number of kills to 6. Since she literary auto-wins the game the requirement MUST be hard, I still think that a rework is better to remove the auto-win completely, but I don't have the exact idea how so a simple nerf will work too.
Vladimir:
Allies can only be damaged by his ability once, every other time they will be "struck for 0 damage".
??? Add lifesteal at lvl 2 and Regeneration at lvl 1 maybe?
The first part simply means that once an ally was damaged by his ability, they won't be damaged again at any point even if they are affected by it again, this will perhaps let Vlad be useful in decks without Scargrounds as he won't be killing his allies as much. Note that even though allies won't take damage more than once, they will still get their "when I survive the damage" perk activated, as they are technically struck. Giving him lifesteal is questionable considering his stats especially that he already drains at lvl 2, but I'd like to see it playtested regardless.
Jarvan IV:
So Jarvan in general is pretty underwhelming and incredibly difficult to level up, despite him having a barrier he is pretty fragile with just 3 health, considering all this armor a Tough keyword could work for him, as well as making him easier to level up but I'm not entirely sure if just reducing the number is a good idea, perhaps rewriting it completely could work, as now the fact that 0 power blockers don't count is pretty annoying.
Also rework LeBlanc maybe so that she actually feels like ya know, LeBlanc?
Other cards:
Soul Sheperd:
Instead of granting, she will give the bonus stats while on the board, so when removed or silenced, the buffs will be gone.
Just to add more counterplay to the aggro ephemeral strats which became more popular now with Azir, possibly reducing her health to 2 or changing the buff she provides to +2|+0 or +1|+0 could work too.
Iterative Improvement:
This will quite simply make it so that it's not as annoying when duping the burblefishes, people will no longer be able to summon a copied follower without passing over the initiative to the opponent.
Wiggly Burblefish:
A simple change that will stop the burblefishe's cost from dropping below 2, another way to limit elusive spam.
Hush and Starshaping:
The most loved 2 Targonian cards both could use the same change, making them both fast really opens up for a lot of potential counters and removes a lot of frustration about the cards. For the hush itself, perhaps even increasing the cost to 4 could be justified.
Atrocity:
So Atrocity only works on followers but is cheaper, this will force players to be more creative than just beefing up Nasus and killing him to win the game, there is so much creative potential for him and other champions which is completely wasted because of this card, cuz why to bother being creative when you can 1 shot the nexus with a single spell. This change will also open up space for Riot to safely make champions with a lot of power without having to worry about this one spell. Also, remember They Who Endure? That archetype was basically replaced by Nasus, but with this change, both of them will have the potential to see play as Nasus is strong enough to be viable without Atrocity, and TWE will have a flat buff to the strat. In general, I think it's much healthier for the game for this spell to only be able to target followers.
Ravenous Flock:
As a 1 cost spell with a pretty big impact, it's pretty easy to enable in the right deck, I think it could use a damage nerf OR a cost increase instead, but it's not as important as the previous changes.
Aaaand that's it from my suggested changes, of course, it's not a whole balance patch, but it's a nice fruit for thought. Remember to treat things here more as an idea, aka "a direction towards which balancing could go but does not have to be exactly the way I described", I'm curious what yall think about my ideas and if they are any good, feedback appreciated! x)