V1
Dictionary <int, PackedScene> ProjectileTypes { get; set; }

ProjectileTypes = new Dictionary<int, PackedScene>()
        {
            {1, FireballScene},
            {2, IceballScene}
        };

Projectile projectileInstance = (Projectile)ProjectileTypes[CurrentProjectileId].Instantiate();



V2
Dictionary<int, Projectile> TestTypes { get; set; }

TestTypes = new Dictionary<int, Projectile>()
        {
            {1, (Fireball)FireballScene.Instantiate()},
            {2, (Iceball)IceballScene.Instantiate()}
        };

Projectile projectileInstance = (Projectile)TestTypes[CurrentProjectileId].Duplicate();