Some good ideas already in already, so I’ll mix them with some of mine.
Sheggra - Change Molten to gain 1 attack on red gem matches. Or, you know, anything other than what it is now. Even fire link would be better than this “legendary” trait. This troop is unsuitable as an attacker having pitiful attack for a legendary, so having two skull traits that inflict the same status effect is pretty lame. Even with a steadily increasing attack, I would consider her not a solid contender for the first or second spot, but in combination with Huge might be slightly more of a threat if your team starts getting picked off. Alternatively, just rebrand her as a full rear line troop, which she is mostly used as now, replace Aflame with Insulated, and replace Molten with “all allies gain 3 attack when I cast a spell” or “all allies gain 1 attack when an ally casts a spell”. OR give her back magic, have her spell target an enemy, dealing double damage if they are burning plus the spawn 6 reds + convert to skulls. OR make her spell deal split damage in addition to the spawns and converts, plus additional fixed damage for every burning enemy (if gone this route, Molten should at least be changed from targeting the first enemy to targeting a random enemy)
Bunni’Nog - Increase mana cost by 1-2 (or maybe don’t), and add “gain an extra turn” to its spell. This guy is basically discount Kerberos except for his last trait, which is ridiculously specialized, while countering pretty much nothing that actually matters. Maybe if we get a Legendary or Mythic knight troop that completely overtakes the meta (no, War does not count, a stiff breeze can already take him out) we might see him more. Fully or partially mana blocking every single skull spawner in the game and complete lack of resistance/reduction traits also don’t help his case for being a viable front liner or even second troop, which his traits and spell seem to imply it is meant to be.
Kerberos - His traits are a mess and nothing really synergies except Arcane. Replace Pyromania with a support trait like a link or Huge OR replace Gate Guardian with something that helps him survive that doesnt involve putting a super low attack troop in front of him (this is even worse now that Ghoul costs more so it mana blocks harder, doesnt gain attack with its spell so it trades skulls even worse, and is not viable to just use as a charge troop anymore since its gem destroy no longer scales with magic). Possibly boost his spell to regain some mana if the enemy dies. If he keeps Pyromania, add burning to his spell.
KoS - Maybe just add deathmark random enemy or two enemies to the spell? (giving it magic and having a minor armor to all allies would also be nice) Spell doesnt need a full rework. Soul legion could be buffed to 100% (maybe even 50% Wight, 50% Wraith). Wraith at least has good lockdown when it gets a skull hit. Soul Legion could also possibly be reworked to a 20-25% summon when an ally dies, which would give it that annoyance walling/factor that IK’s immortal has. If not summons, change his legendary trait to “gain 6 mana when an enemy dies”, which would probably be borderline abusive.
Gar-Nok - Add summon a few skulls to his spell, so at least summoning would give you a better chance to trade hits with your Orc. Move the attack gain to after the summon. Also his traits could probably use a rework. He can get massive attack in the last position… where that attack is usually meaningless, as these knock down drag out fights of last troop vs last troop dont usually happen much at high end. As a summoner type (ie., fairly slow) I dont see him overtaking the high level meta anytime soon, but those couple of changes would help a lot.
Shadow Dragon - Change Shadow Fall trait from attack to magic. This troop isnt really suitable as a front line troop, having no skull resistance, self healing, no Huge, etc. It is also a rather sizable mana block of color it converts to, which is nice since it self charges, but bad because it doesnt usually charge much else with the overflow. He is good at keeping yellow off the board and keeping his own mana up, awaiting your chance to get an extra turn again to re cast, while chipping true damage and poison on the other team, or as an extra purple “battery” used below another purple troop to catch your only yellows. Having its spell get stronger from using it at the right moment rather than attack seems like a better option.
Orion - Damage would need to be lowered if the randomness is removed. It should at least get the Berzerker treatment, having a minimum damage of 3 is a joke since there is no way his spell anything in one shot even if it hits for max, and the average damage is… below average (worse than most commons ascended to legendary, when factoring in Kingdom bonuses). Maybe add a conditional extra turn so he can actually take advantage of that Hunter’s Mark outside of serendipity or using a skull spawner. Jainus’ true damage idea is also nice (again, with more reasonable damage numbers if random was removed).
Gloom Leaf - Keep Corruption’s armor steal scaling with magic. Steal 2 attack from all enemies, and change all brown gems to purple to boost the effect, 5:1 (symbolizing corrupting the earth, synergizes with green Seer). Being able to loop to Green Seer and skull spammers with an extra turn could potentially automatically throw it into the OP category, though. Possibly increase mana cost by 1-2, but his current spell already has one of the worst ROI of any legendary spells, and he is still completely countered by a single Mang or Bone Dragon spell. (if buffed, also fix thorns to only deal damage based on how much was actually dealt to him, since he doesnt have to rely on a gimmick to be good anymore)
Carnex - Buff the armor gain. OR Change armor gain to based off magic, and explode all skulls (or a fixed amount at between 10-15 ish). You’ll generally have more magic than skulls to explode anyway, except at extremely low level. I actually think Carnex is fairly decent currently as an Impervious tank, but that pitifully low fixed armor gain generally makes you lose a lot more armor (and therefore attack, if fully traited) due to a retaliatory skull hit after using his spell if you are using it in this role, so slightly more armor would help mitigate the loss.
Behemoth - Conditional damage buff on it’s spell based on what is destroyed, seconding Jainus here. Instead of destroying gems based on attack, which could probably get a bit out of hand and would require attack buffing troops to take full advantage, having him gain a small amount of fixed attack (2-4) on his spell would also help him out as a secondary tank when paired with his immense trait and synergize with his potential to occasionally set up random cascades with Lay Waste.
Kerberos, perhaps?