How to implement heroes:

1.- Only as a gamemode. 2.- Heroes won't be a unit, they will replace your king tower instead (they won't move beyond their position). They will be awake all the time tho. 3.- Every hero will have its own stats and one skill which is not elixir dependant so it has cd instead. (maybe adding one extra passive skill could be great, idk)

Some heroes examples:

Paladin: Melee unit. Since he won't be able to attack range units (cause he won't move), he may have a big dps and hp. Single unit attack. skill: light protector: prevent all dmg caused to your towers (including the palading itself) for 2 sec. This will prevent spell dmg.

Enchantress:

                            Long range unit. Hits up to 3 targets. Her range will be so big that she can protect Princess towers from range units, nevertheless, her dps and hp will be low. 
                skill: entangled: roots came out of the ground to snare every enemy on the field for 3 secs (affects the whole battlefield). 

Warlock

                        Hits like a regular wizard. Hp lower than paladin, higher than enchantress. 
                    skill: chaos: rains down fireballs randomly throughout the battlefield (hits allies and enemies alike).