Make a character good:

20three

Rules are pretty simple, take a character who is utter, reprehensible garbage and turn them into a good one that people would actually use. For example:

Iron Man 3*

Unibeam 10

Deals 4172 damage (Generates 1 AP for other colors).


Ballistic Salvo 15

Deals 3130 damage and Stuns current target for 4 turns.

Recharge 10
Creates 3 Countdown tiles that activate after 2 turns, restoring 3 Red, 3 blue, 2 green, 2 purple, and 2 black AP each. Iron Man is then stunned for 2 turns as systems reinitialize.

Currently he's absolutely worthless and does no attack well, now he seems borderline good (I think)

ErikPeter

Honestly I think he'd be fine if his powers didn't sap AP and he had tankier HP. Yellow is problematic as well, but AP generation on a usually-passive color is nice.

Recharge 11 AP
Create 3 Countdown tile of each color. After 3 turns, these tiles activate, generating 1 AP of the matching color. Iron Man is stunned for 1 turn while systems recharge.
Rank 2: Create 4 tiles of each color.
Rank 3: Countdown reduced to 2.
Rank 4: Create 3 tiles of each color that generate 2 AP.
Rank 5: Creates 3 tiles of each color that generate 3 AP.

Probably OP, easily generating 6-9 AP in most colors for 11 AP which is a great deal. Plus it creates 18 countdown tiles which will collapse the board, so probably way too good. But I like that it gives the defender some options, e.g. "Oh, better focus on those red tiles so he can't Unibeam..." Anyways it's more food for thought than anything. Just gotta make the power more interesting and less prone to complete failure.

Anyways, on to further thoughts on the original topic.

I don't really think any characters are utter, reprehensible garbage. But here are some tweaks that could make the following guys a bit more playable.

Sentry

Sacrifice
Create 2 strike tiles. (Of the same strength as current.)

Boom, done, he is still nerfed but doesn't blow anymore. At a cost of 1700+ Health, his Strikes should at the very least be noticeably stronger than Black Panther's, which cost 1 more AP but have no drawback, are harder to clear from the board (as there are 3 of them), and become considerably stronger if you have the sufficient TUs.

Wolverine (X-Force)

X-Force
Destroy 9 tiles in an x-shaped pattern and deal 183 damage. (Destroyed tiles do not generate AP.)
Rank 2: Deals 548 damage.
Rank 3: Deals 685 damage and destroys 10 tiles in two X-shaped patterns.
Rank 4: Destroys 15 tiles in three x-shaped patterns.
Rank 5: Deals 1150 damage.

Max level: Deals 2300 damage and destroys 15 tiles in three X-shaped patterns.

This change basically makes rank 4 equivalent to the current rank 5 and gives it a 1K damage buff at max. But more importantly, it makes the first 3 ranks not suck tinykitty. Never have I loathed this game more than when I finally got my second green cover for X-Force and was treated to a whopping 50 damage boost. It's such a joke, and a crummy power until 4 & 5, when it becomes serviceable (but still undeniably worse than his other two powers).

Captain Marvel (Modern)

Hypersonic Punch 8 AP

Energy Absorption
When Captain Marvel takes greater than 695 damage, she generates 3 Red AP.
Rank 2: Generate 4 Red AP
Rank 3: Generate 5 Red AP
Rank 4: Generate 6 Red AP and 3 Black AP
Rank 5: Also stuns the target enemy for 1 turn.

This is a minor buff (Black is cheaper by 1 and Yellow starts at the current rank 2 and gets better) that could make Capt. Marvel closer to the top. Comparing her passive to Hulk's is easy--it's a lot worse. If it stunned the attacker for a turn they'd have to stagger their finishing moves, practically guaranteeing a Photonic Blast or two -- compelling people to whittle her down some and pay more attention to her. It also makes a 5 red build viable (5/4/4 losing 1 turn of stun on black) unlike the current 5-red build which is useful against enemy protect spammers, but awful in all other situations. I'd still probably go 3/5/5 but at least I got to make an interesting choice.

Storm (Mohawk)
Man. How to make Storm better? She used to be sooo good. Now, she's decent with Magneto (despite clashing on yellow) and that's about it.

Mistress of the Elements (Passive)
When your team makes a yellow match, shatter 1 random Team-Up tile, gaining AP.
Rank 2: Shatter 2 random Team-Up tiles.
Rank 3: Also deal 140 damage per tile shattered.
Rank 4: Shatter 3 Team-Up tiles.
Rank 5: Shatter 4 Team-Up tiles, gaining AP and dealing 190 damage per tile.

Seems OP but keep in mind that you'd need to drop covers in Green and/or Black to max this power. Currently her yellow shatters TU and does like, 8 damage... But costs yellow which doesn't mesh with other TU-reliant characters. This way, she can match yellow for Black Panther's Battleplan while accumulating TU to boost the effect. Plus she's shaking up the board which is great and fits her style.

Starsaber

Captain Marvel (both versions)

Change the character names to Carol Danvers (Captain Marvel) and Carol Danvers (Ms. Marvel) to be consistent with Sam Wilson.

As for mechanics:

Photonic Blasts
All tiers keep the same damage as before.
Level 1: Destroys up to 2 random Protect tiles
Level 2: Destroys up to 2 random Protect or Strike tiles
Level 3: Destroys up to 3 random Protect or Strike tiles
Level 4: Destroys up to 3 random Protect, Strike, or Attack tiles
Level 5: Destroys up to 4 random Protect, Strike, Attack, or Countdown tiles.

This would add a bit more versatility to her red, making it more versatile and maybe worth taking up to 5 covers.