If you want classes, you can start with some of the classics, like:
A warior, who try to win with brute force HP and Dmg, the combos units and spells would be all incressing those status, or the armor, or someting like that. Like double damage, and than double again. Also some kind of protector, who would take all the ranged damage insted of other units. You can replace magic books by bravery buffs.
A priest that has a lot of healing spells and units. also units with regen, or units and spells that cast suport effects, like a protect, an armor, one restoration of the amunition, haste that make unit move faster, ressurrection, a unit/spell that does damage by the health, not the attack. And for last a holy nova for damage, things like that make a good priest...
A spelcaster/wizard/witch/mage who ... you know... use magic... and magic units too and combos with magic, like someone who gets stronger everytime you use magic, or a unit that gives back 1 use of magic when it dies, or someone that buff in 1 damage the spells that do damage!
A druid, who transform their units into stroger animals, summon animals and use animals for damage, also poison animals and flying animals. You can replace the magic books by whistle or something to call for animal suport.
A hunter, who mostly use ranged units and ranged itens/spells
you could switch the spells, that traget units, for traps, that traget tiles.
A Barbarian who put everything on offence - units with higth attack, gain rage when less that half hp to hit x2 dmg, rage magic, rage item, itens that gives +attack and - hp, bloodthirsty to give some sustain...
A Bard, who essentialy debuff all the enemies/ buff all the allies. Verry weak units, lots of buffs or debuffs... switch the magics to songs, dances or something. Maybe everyone doing combos with everyone, would be very diferent for sure.
A thief who steal gold, bribe units, debuf foes and conceal units.
An assassin, who has itens that give double attack, stealth, surprise attack, crit, poison hit, death clock... weak units, but cunning, sneaky, and with tricks. Also, you can switch the magics for tricks.
An aprendice/mimic class, who can use units itens and skills from other classes, but not the strogests, maybe the player could choose which of the tier 2 they want as option or something like that.
The better part is: even if the class wasn't verry balanced, It's ok! it would be still funny to play, some are going to be, naturaly, better and easier that others.
bonus! the classes could have some unique trace, like the necromancer who can win even if she loses all the hearts after beating the king, thats because she is the necromance class and only needs to ressurrect herself after all.