adding some pseudocode for ideas

pseudo.js

@@ -0,0 +1,132 @@
+// ./models/validators/isNumerical.validator.js
+export default function isNumericalValidator(propertyNameOrNames, customErrorMessage, callback) {
+  const model = this;
+
+  if (propertyNameOrNames.constructor === Array) {
+		let errorMessages = [];
+
+    const propertyNames = propertyNameOrNames;
+
+		propertyNames.forEach((property) => {
+			if (!isNumerical(model[property])) {
+				const errorMessage = customErrorMessage || `"${property}" must be a number`;
+				errorMessages.push(errorMessage);
+			}
+		});
+
+		if (errorMessages.length === 0) {
+			callback();
+		} else {
+			const error = new Error(`${errorMessages.join(", ")}.`);
+			callback(error);
+		}
+  } else {
+		const propertyName = propertyNameOrNames;
+
+		if (isNumerical(model[propertyName])) {
+			callback();
+		} else {
+			const errorMessage = customErrorMessage || `"${propertyName}" must be a number.`;
+			const error = new Error(errorMesage);
+			callback(error);
+		}
+  }
+}
+
+function isNumerical(value) {
+	const valueIsNumerical = !isNaN(parseFloat(value)) && isFinite(value);
+	return valueIsNumerical;
+}
+
+// ./model/plugins/isProtected.plugin.js
+export default function isProtected() {
+	const model = this;
+
+	model.validations.add("isProtected", isProtectedValidator);
+}
+
+function isProtectedValidator(propertyNameOrNames, customErrorMessage, callback) {
+  const model = this;
+
+  if (propertyNameOrNames.constructor === Array) {
+    const propertyNames = propertyNameOrNames;
+
+    propertyNames.forEach(addFilter, model);
+  } else {
+    const propertyName = propertyNameOrNames;
+
+    addFilter.call(model, propertyName);
+  }
+}
+
+function addFilter(propertyName) {
+  const model = this;
+
+  model.filters.add((newPropertyName, newPropertyValue, callback) => {
+    if (propertyName === newPropertyName) {
+      const errorMessage = customErrorMessage || `${propertyName} is a protected property.`
+      const error = new Error(errorMessage);
+      model.errors.push(error);
+    }
+  });
+}
+
+// ./models/ourModel.js
+include isNumericalValidator from "./validators/isNumerical.js";
+
+export default class OurModel extends Model {
+	beforeInitialize(attributes, options) {
+		// Simple native plugins
+		this.use.timestamps;
+		this.use.softDelete;
+
+		// Simple 3rd-party plugins
+		this.use(isNumerical);
+
+		// Validate one property
+		this.ensure("id").isProtected;
+
+		// Validate many propertyNames
+		this.ensure(["id", "createdAt", "updatedAt", "deletedAt"]).isProtected;
+	}
+}
+
+class User extends OurModel {
+	initialize(attributes, options) {
+		// Immediately begin using 3rd-party functionality
+		this.ensure("age").isNumerical;
+
+		this.ensure("name").isSet;
+		this.ensure("age").isSet;
+
+		super();
+	}
+}
+
+class Admin extends User {
+	initialize(attributes, options) {
+		this.ensure("key").isSet;
+
+		super();
+	}
+}
+
+let user = new User("Bob", 46);
+const admin = new User("Steve", 54, "1209ufajksbdv0q9h3csdv");
+
+user.id = 54;
+user.save(); // OK!
+
+user.id = 42; // OK! You are directly setting the id!
+
+let user = new User({name: "Bob", age: 46, id: 4}); // Doesn't throw, but .save will return error now
+
+user.save((errors) => {
+	errors;
+});
+
+user.errors((errors) => {
+	errors.forEach((error) => {
+		console.log(error.message);
+	});
+});