FloatVariable.cs

using UnityEngine;

[CreateAssetMenu(fileName = "FloatVariable", menuName = "~/Assets/Scripts/FloatVariable")]
public class FloatVariable : Variable<float> { }

IntVariable.cs

using UnityEngine;

[CreateAssetMenu(fileName = "IntVariable", menuName = "~/Assets/Scripts/IntVariable")]
public class IntVariable : Variable<int> { }

StringVariable.cs

using UnityEngine;

[CreateAssetMenu(fileName = "StringVariable", menuName = "~/Assets/Scripts/StringVariable")]
public class StringVariable : Variable<string> { }

Variable.cs

using UnityEngine;
using System;

public class Variable<Element> : ScriptableObject, ISerializationCallbackReceiver
{
    public Element initialValue;
    public GameEvent onRuntimeValueChanged;

    [NonSerialized]
    private Element _runtimeValue;
    public Element runtimeValue
    {
        get { return _runtimeValue; }
        set
        {
            _runtimeValue = value;
            if (onRuntimeValueChanged == null) return;
            onRuntimeValueChanged.Raise();
        }
    }

    public void OnAfterDeserialize()
    {
        runtimeValue = initialValue;
    }

    public void OnBeforeSerialize() { }
}