If you finish the game, you have at the end 25 stars left.
Many persons would like to have a lightning spell tree.
So the additional stars could be spend for the tree and maybe for the hero abilities, otherwise the hero may not use them, if he levels up.
I would also suggest a third ability, that every hero gets if he reaches level 3, 6 and 9 (because the other abilities ar obtained at level 2,5,8 and 4,7,10).
So, each ability could cost 6 stars, 7 remaining for the lightning tree.
Here are my suggestions for the abilities:
I would add to each hero a passive-like ability.
I took a closer look at the names of the heroes and it was easy to find new ones, i kept in mind, they shouldn't be too powerful and fit to the hero, maybe also create synergy effects with the other abilities.
Bolin: He is called Farslayer, why shouldn't he get with every level a slight range boost.
He is slower then Alleria and Magnus, so he has not to move as much
In addition to that: It annoys me, that the mines are so small and therefore they miss often their target. It could help, if he could shoot from a far distance, lay mines, shoot, lay mines and so on, until he has to move away, so he doesn't have to engage the enemy in melee.
Until the enemy has reached him, he can attack much more often and lay more mines.
Alleria: If she is idle, she becomes invisible until she attacks for the first time.
I think it would be good and fit to her theme (Ranger and Swiftwind) if she moves faster, as she is invisible and then her first attack could also do twice the normal damage. The damage and speed could be increased, at each level gain.
Ignus: Ignus moves really cool and leaves small flames on the ground. The flames could stay on the ground and slightly damagy the enemys, that walk over them. Like the ground, after you used the meteor. The damage shouldn't be too high, so it should be better to attack with Ignus, then to run in front of the enemy^^ The duration and damage of the flames could be increased with each level.
King Denas: I am not quite sure about this guy. I like that he buffs your towers and the king could take less taxes ;P
So at level 3 your tower costs (also the costs to upgrade them) is reduced by 5%, at level 6 10% and at level 9 15%.
But maybe it is not a good idea.
Magnus: Magnus Spellbane... He could curse the enemy if he attacks. Maybe he also just has a chance to curse the enemy, that could be increased with each ability-level.
He represents the mage-towers, but looks and feels like an arcane mage.
Okay, he is probably an arcane mage, but i think it would be cool, if he could curse the enemy like the elemental tower does.
The enemy would take each time you attack him additional damage, 3/6/10 dependent on the level.
He can teleport, so it would create a synergy effect with this "ability", because he could teleport to the next enemy, after he cursed one.
Malik, the Hammerfury: If he gets hit by an enemy, his damage and attack speed are increased by 33/66/100 percent for 1/2/3 seconds.
That could help to level him faster, but this ability would be probably too powerful.
Sir Gerald: He can use an aura/buff the own troops, why not use an aura to debuff the enemy?!
Each ability-level increases the range of the aura (circle around Sir Gerald) and adds additional effects.
For example, the abilitys of the Demon Lords (the Shields) and the Buff of the Troll Drummers are nullified...
If the enemy in the circle uses a magic attack, it is counted as normal attack, no armor piercing...
Spiders may not lay eggs, he could nullify the abilities of the enemy, rocket packs, the summon-ability of necromants, etc.
That ability is probably difficult to implement...
So, this are my suggestions, don't be too harsh to me^^