Dynamic spells and keybindings

So, I'm building a dungeon crawler with 4 bindable spell slots. Each slot can have any of the spells in game.

I was thinking of working with inheritance, having one main spell which all other spell derives from, but as I need the script to have the MonoBehaviour, I can't do that.

As it is right now I have a prefab for each spell with it's script on it, I then instantiate that prefab when I "cast" a spell, but this means that I can't make the keybindings dynamic as I can't cast these spells from one line on code.

I hope you understand my question

I would like to have something that says

 public Spell firstSpell;
 firstSpell = new ProjectileSpell();
 
 public void Cast()
 {
             if(firstSpellReady)
             {
                 Cast(firstSpell);
                 firstSpellReady = false;
                 firstSpellCooldown = firstSpell.timer;
             }
 }

Or something that I haven't thought of that works better in this situation.